As mentioned in Chapter 3, all industrial control systems can usually be structured into a set of hierarchical levels. Figure 3.1 gives a definition of such a set; the management level, the control level, the field level, and the equipment level. Similarly to computer networks, communication can take place on several hierarchical levels, implemented by means of special devices termed interfaces.
In this definition, the network of field devices such as sensors and actuators is defined as the field level. In fact, the field level includes the systems which connect the field devices with the programmable controllers in order to permit communication between them. Communication at field level are obviously necessary to real-time control and automation because the process states and device statuses must be made available to the various, often remotely distributed, programmable controllers and visualization stations. Safe and reliable communications at the field level are therefore essential.
There are three types of interfaces offering real-time process control and production automation: the Actuator Sensor Interface (AS-Interface, or AS-i), the highway addressable remote transducer (HART), and fieldbuses.
HART communication uses conventional 4 to 20 mA current loop for data transmission, providing a very simple point-to-point connection between operating and field devices without needing additional wiring. The HART protocol is therefore useful when smart field devices, such as intelligent and programmable sensors and actuators, are to be integrated into an already existing plant. With the appropriate instrumentation, however, HART is also suitable as a communication system for extended plants. The only prerequisite is that the field devices are connected according to the conventional 4 to 20 mA technique.
Fieldbuses are wired completely differently, replacing the analog 4 to 20 mA current loops with a simple two-wire line, running from the control station to the field connecting all devices in parallel. Information transmission is purely digital, including control and process monitoring information as well as the commands and parameters required for start-up, device calibration and diagnosis.
When only binary (on/off) states need to be transmitted, the relevant system components can be networked via a simplified bus system. For applications in hazardous areas, the open bus system AS- Interface is a good solution. If required, this can be integrated via a special connection into more powerful fieldbus systems.
Apart from the above, two other solutions are avilable, adopting a middle course. In both cases, the field devices are wired according to the conventional 4 to 20 mA technique, but the lines are not run up to the control station because the signals are digitized before being supplied to a bus system. This task is accomplished by the field multiplexer. When the digital-analog conversion takes place in the control room, the system is called rackbus, whereas conversion in the field is performed by a remote I/O system.
Industrial automation systems require large numbers of control devices, often including a number of binary (on/off) actuators and sensors. Conventional input and output (I/O) methods for wiring include point-to-point connections or bus systems. Typical batching valve wiring networks attach each of the I/O points to a central location, resulting in multiple-wire runs for each field device, which is expensive, and requires a considerable amount of space. These methods can prove to be too complex for networking simple binary devices and too slow for the interactions between the controllers and the controlled devices. Point-to-point wiring is the most common method of wiring in industry, but large wire bundles take up valuable space, installation is time-consuming, and troubleshooting is complex.
The Actuator Sensor Interface, or AS-Interface or AS-i, was developed by a group of sensor manufacturers and introduced into the market in 1994. Since that time, it has become the standard for discrete sensors and actuators in process industries around the world. It is also a bus system, used for low-level field applications in industrial automation to communicate with small binary sensors and actuators using the AS-Interface standard. This modernizes automation systems effectively and eliminates wire bundles completely, with only one wire cable required for all devices, compared to one cable from each device needed for point-to-point wiring. Junction boxes are also eliminated, and the size of the control cabinet needed is significantly reduced. Plug-and-play wiring supports all typologies. Figure 12.1 gives the locations of the AS-Interface in an industrial control network.
When compared with conventional I/O wiring methods, the AS-Interface has many advantages. The most important ones are:
1. Minimum wiring giving cost-savings. The AS-Interface needs a single cable, which uses simple serial connection to the controller, instead of parallel with a multitude of cables.
2. Fast and safe installation. Sensors and actuators are simply installed with modules on the AS- Interface cable. Contact pins in the modules penetrate the insulation of the cable and establish contact with the copper wire. Incorrect connections are practically impossible because of the design of the cable and the special piercing method.
3. Flexible configuration. Owing to the distributed and modular design, plant sections can be tested in parallel even before the overall solution is finished. This permits flexible modification and expansion.
4. Open system. AS-Interface is an open system, which means that it is independent of manufacturer and future-proof.
12.1.1 Architectures and components
There are two types of AS-Interface architectures as displayed in Figure 12.1.
(1) AS-Interface architecture: type 1
In the first type of AS-Interface architecture, a programmable controller such as a PLC, SCADA, or PC controls sensors and actuators via field-level buses, including Foundation Fieldbus, Profibus, etc.
As displayed in Figure 12.2, the AS-Interface has gateways directly connected to the field-level bus and the I/O module. The latter is the device which contacts the sensors and the actuators. A field-level bus may be able to support several AS-Interface gateways depending on the manufacturing specification and the system design, each of which fits a segment of an industrial control system.
In this type of architecture, the AS-Interface requires the following components:
Gateways are devices that interface between the AS-Interface and a higher-level bus system. They are used when more complex applications are to be implemented using standard products. AS-Interface gateways are the core of the wiring system, handling the complete data transfer, by cyclically polling all participants (master/slave) connected to the wiring system. They can be placed anywhere in the AS- Interface segments. At present, one gateway routinely handles 124 inputs and 124 outputs over 31 addressable I/O modules, and setup is accomplished through the setup tools of the respective system.
(2) I/O modules
I/O modules are located between standard sensors and actuators and the AS-Interface. They are available for many kinds of application, including flat modules for limited-space applications, compact modules for a variety of mounting options, field modules that use cord grips instead of quick disconnects, and standard modules that use both the mechanically keyed AS-Interface cable and the standard 16 AWG round cable. For enclosures and junction boxes, enclosure modules connect the AS-Interface bus to a power rail system, and junction box modules for use within junction boxes.
(3) Power supplies and repeaters
In an AS-Interface, one single cable transmits both power and data. Power supplies contain internal data separation coils so that the capacitive filtering of the supply does not interfere with the data stream. Adding to the high interference immunity of AS-Interface is the power supply data isolation coil between the voltage transformer and the output so that the data signals are isolated from line noise. Repeaters extend AS-Interface networks by up to 100 meters; by using two in series an AS-Interface network can be up to 300 meters long. Repeaters do not require a network address and allow I/O modules to be placed anywhere along the network.
(4) AS-Interface safety at work
This extension of the AS-Interface allows safety equipment to be wired together on a two-wire cable rather than hardwired back to a panel. A maximum of 31 category 4 inputs, such as E-Stops, can be put on one cable. The parts included in this section are safety slaves, safety monitor, and configuration software.
In many applications, safety-relevant functions are a prerequisite. They take the form of emergency stop buttons near process lines, or the installation of safe sensors (e.g., safe photo grits and locking of safety-related doors) to automatically stop machines.
Integration of the safety requirement into the AS-Interface line through the terminology “AS- Interface safety at work”, can drastically reduce additional costs. The concept refers to connection of the safety-related switches by a safe AS-Interface module. There is also a safety monitor on the AS- Interface line that permanently observes the communication. The communication happens through a given and predetermined pattern in a dynamic code table with an 8 times 4 bits sequence. The safe monitor continuously compares “must” and “actual” values of the communication. In the case of the bit sequence 0000, the safe monitor switches off the safe relay in less than 40 milliseconds. Several safe monitors can be operated in one AS-Interface line arranged in any position.
Clear benefits of AS-Interface safety at work include the following: only one AS-Interface line is required for the communication of safe and non-safe data; full compatibility with all standard AS-Interface devices; no specific communication mechanisms required; mixed applications on one and the same AS-Interface line possible; diagnosis of the safe modules via the standard AS-Interface master possible.
(5) AS-Interface encoders
In order to be able to meet the real-time requirements of many applications, a multiple-slave solution can be adopted. The position value, up to 16 (or 32) bits in length, is transferred to the gateways within a single cycle, via the four integrated AS-Interface chips used for control purposes. AS-Interface rotary encoders include 13-bit-Singleturn and 16-bit-Multiturn.
To complete the AS-Interface and to make the installation as easy as possible, various accessories such as hand-held addressing devices, mounting bases, simulators for higher-level bus systems, and sealing for flat cable and adaptor to round cable are available.
(2) AS-Interface architecture: type 2
In the second type of AS interface architecture, as shown in Figure 12.3, the AS-Interface master module resides inside a programmable controller such as a PLC, SCADA, or PC. The AS-Interface master terminal enables the direct connection of either analog AS-Interface slaves or digital. It does not manage the sensors and actuators via the field-level buses, but rather via the AS-Interface slave modules or cables. The modules are connected to each other by means of the AS-Interface cable, which can be branched with the cable branch device. A group of slave modules frames a segment of an
industrial control network with one interface cable. The AS-Interface master module is able to support several segments depending on the designed system capabilities.
In this type of architecture, AS-Interface requires the following components.
(1) AS-Interface masters
The AS-Interface master automatically controls all communication over the AS-Interface cable without the need for special software. The master can connect the system to a programmable controller such as a PLC, SCADA, or PC, act as a standalone controller, or serve as a gateway to higher-level bus systems. The following AS interface masters are currently available:
(a) Standard AS-Interface master. Up to 31 standard slaves or slaves with extended addressing mode can be attached to standard AS-Interface masters.
(b) Extended AS-Interface masters. They support 31 addresses that can be used for standard slaves or those with extended addressing mode. AS-Interface slaves with extended addressing mode can be connected in pairs (programmed as A or B slaves) to an extended AS-Interface master and can use the same address. This increases the number of addressable AS-Interface slaves to a maximum of 62. Due to this address expansion, the number of binary outputs is reduced to three per AS- Interface slave.
(2) AS interface slaves
All the nodes that can be addressed by an AS-Interface master are defined as AS-Interface slaves.
(a) AS-Interface slave assembly system. The following assembly systems are available:
(i) AS-Interface modules. AS-Interface modules are AS-Interface slaves to which up to four conventional sensors and up to four conventional actuators can be connected. The standard coupling module, which is the lower section of a standard device, connects the user module to the yellow AS-Interface cable. The user module connects the sensors and actuators, while the application modules connect via screw terminals or connectors. Sensors and actuators with a built-in AS-Interface chip can be directly connected to the AS-Interface cable.
(ii) Sensors/actuators with an integrated AS-Interface connection. Sensors/actuators with an integrated AS-Interface connection can be connected directly to the AS-Interface.
(b) Addressing modes are as follows:
(i) Standard slaves. Standard slaves each occupy one address on the AS-Interface. Up to 31 standard slaves can be connected to the AS-Interface.
(ii) Slaves with an extended addressing mode (A/B slaves). These can be operated in pairs at the same address with an extended AS-Interface master. This doubles the number of addressable AS-Interface slaves to 62.
One of these AS-Interface slaves must be programmed as an A slave using the addressing unit and the other as a B slave. Due to the address expansion, the number of binary outputs is reduced to three per AS- Interface slave. Slaves can also be operated with a standard AS-Interface master. For more detailed information about these functions, refer to the AS-Interface master discussion in the previous paragraphs.
(c) Analog slaves. Analog slaves are special AS-Interface standard slaves that exchange analog values with the AS-Interface master, and require special program sections to execute the sequential transfer of analog data. Analog slaves are intended for operation with extended AS-Interface masters. The extended AS-Interface masters handle the exchange of analog data with these slaves automatically. No special drivers or function blocks are required in the user program.
(3) Further AS-Interface system components
Further AS-Interface components include the AS-Interface cable, the AS-Interface power supply unit, addressing unit, and SCOPE for AS-Interface.
(a) AS-Interface cable. The trapezoidal AS-Interface cable is recommended over standard two-wire round cable for quick and simple connection of slaves. It is available in different colors to signify its voltage rating, with color assignments as follows:
(i) Yellow is used for data and control power between the master and its slaves.
(ii) Black is the external output power cable up to 60 V DC.
(iii) Red is the external output power cable up to 240 V AC.
The AS-Interface cable, designed as an unshielded two-wire cable, transfers signals and provides the power supply for the sensors and actuators connected using AS-Interface modules. Networking is not restricted to one type of cable. If necessary, appropriate modules or T pieces can be used to change to a simple two-wire cable.
(b) AS-Interface power supply unit. The AS-Interface power supply unit supplies power to the AS- Interface nodes connected to the AS-Interface cable. For actuators with particularly high power requirements, the connection of an additional power supply may be necessary (e.g., using special application modules). Data and control power are normally transmitted simultaneously via the AS-Interface cable. Power for the electronics and inputs is supplied by a special AS-Interface power supply that feeds a symmetrical supply voltage into the AS-Interface cable via a data-decoupling device.
(c) Addressing unit. The addressing unit allows simple programming of AS-Interface slave addresses.
(d) SCOPE for AS-Interface. SCOPE for AS-Interface is a monitoring program for Windows that can record and evaluate the data exchange in AS-Interface networks during the commissioning phase and during operation. SCOPE for AS-Interface can be operated on a PC under Windows in conjunction with an AS-Interface master communications processor.
Principles and mechanisms
The AS-Interface utilizes a single, trapezoidal, unshielded two-wire cable, which eliminates the extensive parallel control wiring required with most installations. In a network with the AS-Interface, a simple gateway interfaces the network into the field communication bus. Data and power are transferred over the two-wire network to each of the AS-Interface compatible field devices. The existing controller sees AS-Interface as remote I/O; therefore, AS-Interface connects to the existing network with minimal programming changes.
The AS-Interface system utilizes only one master per network to control the exchange of data. This allows the master to interrogate up to 31 slaves and update all I/O information within 5 ms (10 ms for 62 slaves). For slave connection, an insulated two-wire cable is recommended to prevent reversing polarity. The electrical connection is made using contacts that pierce the insulation of the cable, contacting the two wires, thus eliminating the need to strip the cable and wire to screw terminals. For data exchange to occur, each slave must be programmed with an address that is stored internally in nonvolatile memory and remains even after power is removed.
(1) How the AS-Interface functions
The AS-Interface or AS-Interface system operates as outlined below:
1. Master-slave access techniques. The AS-Interface is a single-master system. This means that there is only one master per AS-Interface network to control the operations of process. This polls all AS- Interface slaves one after the other and waits for a response.
2. Electronic address setting. The address of an AS-Interface slave is its identifier, set only once within an AS-Interface system, either by using a special addressing unit or by an AS-Interface master. The address is always stored permanently on the AS-Interface slave.
3. Operating reliability and flexibility. The transmission technique used (current modulation) guarantees high operating reliability. The master monitors the voltage on the cable and the transferred data. It detects transmission errors and the failure of slaves and sends a message to the controller. The user can then react to this message. Replacing or adding AS-Interface slaves during normal operation does not affect communication with other AS-Interface slaves.
(2) Master-slave principle
The AS-Interface operates on the master-slave principle as discussed earlier. This means that the AS- Interface master connected to the AS-Interface cable controls the data exchange with the slaves via the interface to the cable of the AS-Interface.
Figure 12.4 illustrates the two interfaces of the AS-Interface master communication processor. The first interface is between the master CPU and the master communication processor; the second is between the master communication processor and AS-Interface cable. Process data and parameter assignment commands are transferred via the first interface, and user programs have suitable function calls and mechanisms available for reading and writing via this interface. On the other hand, infor- mation is exchanged with the AS-Interface slaves via the second interface between the master communication processor and AS-Interface cable.
(1) Tasks and functions of the AS-Interface master
The AS-Interface master specification distinguishes masters with different ranges of functions known as a profile. For standard AS-Interface masters and extended AS-Interface masters, there are three different master classes (M0, M1, M2 for standard masters, and M0e, M1e, M2e for extended masters). The AS-Interface specification stipulates the functions that a master in a particular class must be able to perform. The profiles have the following practical significance:
(a) Master profile M0/M0e. The AS-Interface master can exchange I/O data with the individual AS- Interface slaves. The station configuration on the cable, called the expected configuration, is used to configure the master.
(b) Master profile M1/M1e. This profile covers all the functions according to the AS-Interface master
(c) Master profile M2/M2e. The functionality of this profile corresponds to master profile M0/M0e, but the AS-Interface master can also assign parameters to the slaves. The essential difference between extended AS-Interface masters and standard AS-Interface masters is that they support the attachment of up to 62 AS-Interface slaves using the extended addressing mode. Extended AS-Interface masters also provide particularly simple access for AS-Interface analog slaves complying with profile specifications.
(2) How an AS-Interface slave functions
The following describes some slave functions. For more detailed information on the ID codes, refer to the manufacturer’s description.
(a) Connecting to the AS-Interface cable. The slave has an integrated circuit (AS-Interface chip) that provides the attachment of an AS-Interface device (sensor/actuator) to the common bus cable to the master. The integrated circuit contains four configurable data inputs and outputs, and four parameter outputs. The operating parameters, configuration data with I/O assignment, identification code, and slave address are stored in additional memory (e.g., EEPROM).
(b) I/O data. Data that were transferred to the AS-Interface slave are available at the data outputs.
Values at the data inputs are made available to the master when the slave is polled.
(c) Parameters. Using the parameter outputs of the slave, the AS-Interface master can transfer values that are not interpreted as simple data. These parameter values can be used to control and switch over between internal operating modes of the sensors or actuators. It could, for example, be possible to update a calibration value during the various operating phases. This function is possible with slaves that have an integrated AS-Interface connection providing they support the function in question.
(d) Configuration. The input/output configuration (I/O configuration) indicates which data lines of the AS-Interface slave are used as inputs, outputs, or as bidirectional outputs, and can be found in the description of the AS-Interface slave. In addition to the I/O configuration, the type of AS-Interface slave is specified by an identification code; newer AS-Interface slaves are identified by three identification codes (ID code, ID1 code, ID2 code).
(3) Data transfer
Data transfer is one of the most important functions in the AS-Interface operation, as explained below.
(1) Information and data structure
Before introducing the operating phases, and their functions, a brief outline of the information structure of the AS-Interface master/slave system is necessary. Figure 12.5 shows the data fields and lists of the system as configured in the system structure diagram that was given in Figure 12.4. The following structures are found on the AS-Interface master:
(a) Data images. These contain temporarily stored information:
(i) actual parameters that are an image of the parameters currently on the AS-Interface slave;
(ii) actual configuration data that contains the I/O configurations and ID codes of all connected AS-Interface slaves once these data have been read from the AS-Interface slaves;
(iii) the list of detected AS interface slaves (LDS) that specifies which AS-Interface slaves were detected on the AS-Interface bus;
(iv) the list of activated AS-Interface slaves (LAS) that specifies which AS-Interface slaves were activated by the AS-Interface master. I/O data are only exchanged with activated AS-Interface slaves.
(b) I/O data. The I/O data are the process input and output data.
(i) Expected configuration data are selectable comparison values which allow the configuration data of the detected AS-Interface slaves to be checked.
(ii) List of permanent AS-Interface slaves (LPS) that specifies which slaves are expected on the cable by the master. The AS-Interface master checks continuously whether all the slaves specified in the LPS exist, and whether their configuration data match what is expected. The AS-Interface slave has the following structures: (a) I/O data; (b) parameters; (c) actual configuration data; the configuration data include the I/O configuration and the ID codes of the AS-Interface slave; (d) address. Slaves have an address of 0 when installed, so to allow data exchange, their addresses must be set. The address 0 is reserved for special functions.
(2) The operating phases
Figure 12.6 illustrates the individual operating phases.
(a) Initialization mode. Also known as the offline phase, this sets the basic status of the master. The module is initialized after switching on the power supply or following a restart during operation. During the initialization, the images of all the slave inputs and the output data from the point of view of the application are set to 0 (inactive). After switching on the power supply, the configured parameters are copied to the relevant field so that subsequent activation uses the preset parameters. If the AS-Interface master is reinitialized during operation, the values from the parameters field that may have changed in the meantime are retained.
(b) Start-up phase.
(i) Detection phase. During start-up or after a reset, the AS-Interface master runs through this phase, during which it detects which slaves are connected and what type they are from their configuration data. Configuration files contain the I/O assignment and the slave type (ID codes). The master enters detected slaves in the list of detected slaves (LDS).
(ii) Activation phase. After the slaves are detected, the master sends a special call which activates them. When activating individual slaves, a distinction is made between two modes on the AS- Interface master:
Master in the configuration mode: all detected stations (with the exception of the slave with address 0) are activated. In this mode, it is possible to read actual values and to store them for a configuration.
Master in the protected mode: only the stations corresponding to the expected configuration stored on the AS-Interface master are activated. If the actual configuration differs from this, the AS-Interface master indicates this.
The master enters activated AS-Interface slaves in the list of activated slaves (LAS).
(iii) Normal mode. On completion of the start-up phase, the AS-Interface master switches to the normal mode.
(iv) Data exchange phase. In the normal mode, the master sends cyclic data (output data) to the individual AS-Interface slaves and receives their acknowledgment messages (input data). If an error is detected during transmission, the master repeats the appropriate poll.
(v) Management phase. During this phase, all existing jobs of the control application are processed and sent. Possible jobs are, for example:
Parameter transfer: four parameter bits (three parameter bits with AS-Interface slaves with the extended addressing mode) are transferred to a slave and are used, for example, for a threshold value setting.
Changing slave addresses: this function allows the addresses of AS-Interface slaves to be changed by the master if the AS-Interface slave supports this particular function.
(vi) Inclusion phase. In the inclusion phase, newly added AS-Interface slaves are included in the list of detected AS-Interface slaves and, providing the configuration mode is selected, they are also activated (with the exception of slaves with address 0). If the master is in the protected mode, only the slaves stored in the expected configuration of the AS-Interface master are activated. With this mechanism, slaves that were temporarily out of service can be included again.
(3) Interface functions
Various functions are available on the interface to control the master and slave interaction from the user program. The possibilities are explained below. The possible operations and the direction of data flow are illustrated in Figure 12.7.
(a) Read/write. When writing, parameters are transferred to the slave and to the parameter images on the communication processor; when reading, parameters are transferred from the slave or from the communication processor parameter image to the CPU.
(b) Read and store (configured) configuration data. Configured parameters or data are read from the nonvolatile memory of the communication processor.
(c) Configure actual. When reading, the parameters and configuration data are read from the slave and stored permanently on the communication processor; when writing, the parameters and configuration data are stored permanently on the communication processor.
(d) Supply slaves with configured parameters. Configured parameters are transferred from the nonvolatile area of the communication processor to the slaves.
(4) Extended AS interface slaves with standard AS-Interface masters
The following information is about operating extended AS-Interface with standard AS-Interface masters.
(a) Slaves are connected to standard masters. The most significant slave bit (bit 4) of each A slave must be set to 0. The most significant parameter bit (bit 4) must also be set to 1 (default value). Without these settings, the A slave cannot be operated with a standard master.
(b) B slaves must not be connected to standard AS-Interface masters.
Systems and environments
As previously explained, the AS-Interface is an open, non-proprietary bus system. Both actuators and sensors are connected to programmable logic controllers using a non-shielded 2-wire cable on which the data and power are simultaneously transferred.
(1) AS-Interface system characteristics
The most important physical characteristics of the AS-Interface and its components are as follows:
1. Two-wire cable for data and power supply. A simple two-wire cable can be used, supplying both data and power. The power available depends on the supply unit used. For optimum wiring, mechanically coded AS-Interface cable is available, which prevents connections from being reversed and makes simple contact with the AS-Interface application modules using the penetration technique.
2. Tree structure network with a cable. The tree structure of the AS-Interface allows any point on to
a cable section to be used as the start of a new branch.
3. Direct integration. Practically all the electronics required for a slave have been integrated on to a special integrated circuit, allowing the AS-Interface connector to be integrated directly in binary (on/off) actuators or sensors.
4. Increased functionality, more uses for the customer. Direct integration allows devices to be equipped with a wide range of functions. Four data and four parameter lines are available, so the resulting “intelligent” actuators/sensors have many possible uses; for example, monitoring, parameter assignment, wear or pollution checks, etc.
5. Additional power supply for higher power requirements. An external source of power can be provided for slaves with a higher power requirement.
(2) AS-Interface system limits
The AS-Interface system is currently subject to the limits below.
(1) Cycle time
(a) Maximum 5 ms with standard AS-Interface slaves.
(b) Maximum 10 ms with AS-Interface slaves that use the extended addressing mode. AS-Interface uses constant message lengths. Complicated procedures for controlling transmission and identifying message lengths or data formats are not required. This makes it possible for a master to poll all connected standard slaves within a maximum of 5 ms and to update the data on both the master and slave. If only one AS-Interface slave using the extended addressing mode is located at an address, this slave is polled at least every 5 ms. If two extended slaves (A and B slave) share an address, the maximum polling cycle is 10 ms. (B slaves can only be connected to extended masters.)
(2) Number of connectable AS-Interface slaves
(a) Maximum of 31 standard slaves.
(b) Maximum of 62 slaves in the extended addressing mode. AS-Interface slaves are the input and output channels of the AS-Interface system. They are only active when called by the AS- Interface master, triggering actions or transmit reactions when commanded. Each AS-Interface slave is identified by its own address (1 31). A maximum of 62 slaves using the extended addressing mode can be connected to an extended master. Pairs of slaves occupy one address; in other words, the addresses 1 31 can be assigned to two extended slaves per slot. If standard slaves are connected to an extended master, these occupy a complete address; in other words, a maximum of 31 standard slaves can be connected to an extended master.
(3) Number of inputs and outputs
(a) A maximum of 248 binary inputs and outputs is possible with standard modules.
(b) A maximum of 248 inputs and 186 outputs if the extended addressing mode is used. Each standard AS-Interface slave can receive and send 4 bits of data. Special modules allow each of these bits to be used for a binary actuator or a binary sensor meaning that an AS-Interface cable with standard AS-Interface slaves can have a maximum of 248 binary attachments (124 inputs and 124 outputs). All typical actuators or sensors can be connected in this way. The modules are used as distributed inputs/outputs. If modules with the extended addressing mode are used, a maximum of 3 inputs and 3 outputs is available per module; in other words a maximum of 248 inputs and 186 outputs can be operated with modules using the extended addressing mode.
(3) AS-Interface in a real-time environment
The system characteristics listed below means the AS-Interface can work in a real-time environment.
1. Optimized system for binary sensors and actuators and for simple analog elements.
2. Master-slave principle with cyclic polling.
3. Tree structure of the network.
4. Both data and power supplied by means of one unshielded two-wire cable.
5. Flat cable for contacting by piercing technology.
6. Modules act as remote I/O ports for conventional sensors and actuators.
7. Integrated slaves with their own AS-Interface capabilities.
8. No communication software in the slaves, only firmware in the self-configuring master.
9. Low costs, simple installation, easy handling, flexible networks, high reliability in an industrial environment, open and internationally accepted system that has many manufacturers and products.
There are three aspects of the AS-Interface that are of particular importance in real-time applications: connectivity, cycle time, and availability.
AS-Interface has two distinct ways to connect to the first control level.
The first and most important is a direct connection like the type 2 AS-Interface architecture given in section 12.1.1. In this case, the system’s master is an embedded part of a programmable controller such as a PLC, SCADA, or PC, running at its own cycle time. As the AS-Interface is an open system, a manufacturer of any kind of programmable controller can build a master for their own system; many are already available, with more under development.
The second way is to connect AS-Interface via a coupler to a higher fieldbus and to use it as a subsystem, which is the type 1 AS-Interface architecture explained in section 12.1.1. In this case, all data from the AS-Interface network are handled in one node of the fieldbus, and this is connected to the above-lying host together with other components of the higher fieldbus. The application program (user program) handles all data for the particular fieldbus. For real-time applications, an analysis of the cycle time and the availability of the combination of the two systems has to be done. The AS-Interface offers couplers to most known higher fieldbuses such as Profibus, CAN, etc., with others (e.g. LON, Fieldbus Foundation) under development. Together with its tree structure, AS-Interface thus offers the most flexible networking solution to any automation application.
482 CHAPTER 12 Field interfaces
(2) Cycle time
AS-Interface is a single-master system with cyclic polling. Thus, any slave is addressed in a definite time period.
For a complete network with 31 slaves, the cycle time is 5 ms; less with fewer slaves. (With very few slaves the cycle time can be shortened to less than 500 ms.) Analog data with more than 4 bits need several cycles, but do not affect the basic cycle time for binary sensors and actuators.
The cycle time includes all steps from and to the interface to the host system, and even includes one repetition. Data exchange with the host uses process I/O images at the end of each cycle stored in, for example, a dual-ported memory at the interface. Therefore no other steps are needed, for a direct connection to the control device.
This is asynchronous coupling, and in real-time applications this may present a restriction, but for
many systems and applications this is short enough.
Availability in this context means that a system will deliver reliable data and diagnostic values continuously and in time under all specified conditions, especially under severe electromagnetic noise. The answers to three questions are of special importance for real-time applications:
(a) Can electromagnetic noise or other faults disturb the reliability of data?
(b) How much time is necessary for the correction of a faulty transmission?
(c) How often does such a fault happen and can this affect the whole system?