SYSCON MICRO INTERFACING
The syscon microprocessor is a wholly digital device and deals with all inputs and outputs in binary form. Since most commands and feedback indications appear as on-off signals there is no difficulty in matching them to the requirements of the chip – beyond the need for the port expansion, strobe or codec systems already described.
Most syscon outputs, too, are easy to apply to their recipients; motor commands merely toggle electronic switches in motor drive amplifiers (MDAs) and signal-routeing is also carried out by ‘local’ electronic switches. The current-sourcing capability of a microproces- sor port is not sufficient to drive any ‘end-user’ beyond a fluorescent display panel, however, so for indication-LEDs, relays and solenoids, buffer stages incorporating further ICs and/or transistors are used; they have the additional advantage of protecting the micro from electrical damage; providing ‘fan-out’ capability; and if required the facility for logic inversion. Relays and solenoids have special require- ments where they may be held in for long periods – the required pull-in and hold-in currents are greatly different, and can be catered for by two separate windings: a heavy, high-current one is momentarily pulsed to pull-in, then a low ‘hold’ current is maintained through the other. In some custom-designed syscon micros, separate pull and hold output pins are provided. Alternatively an RC charging circuit provides the initial pull pulse. In very energy-conscious portable video systems latching relays and solenoids are used, in which a magnetic armature holds the selected position after a single ‘pull’ pulse in the right direction through the coil.