SCART SYSTEM AND D2B BUS
Details of the SCART plug/socket connector system are given in Chapter 24. There are two control systems associated with it; the simplest consists of a source switching line at pin 8. By raising a high (+12 V) on this line a source-peripheral (tape- or disc-player, satellite converter, computer etc.) can automatically switch the vision and sound circuits of the TV receiver/monitor to baseband operation in order to transfer signals at CVBS or RGB (plus sound) via the appropriate SCART signal pins. Raising SCART pin 8 to an intermediate level (+6 V) indicates a widescreen programme, and can be used in a TV set to switch scanning standards accordingly. The second control system – on SCART pins 10 and 12 – is more comprehensive, and has much in common with the I2C bus already described, though the serial data flow is slower, and includes a security system to overcome the effects of possible data corruption. This control system is called D2B (Domestic Data Bus), and can be interfaced with I2C (and associated cordless remote control systems) by special transcoding chips.
The two D2B data lines take the form of a floating differential pair, in which logic 1 corresponds to a level above 100 mV and logic 0 to a level below 50 mV. the information is transferred at one of three standard speeds, 110, 2400 and 8300 characters per second, based on clock frequencies of 0.554, 2.217 and 4.436 MHz. The signalling is bi-directional and self-synchronising, with an arbitration system similar to that of I2C. To satisfy the auto-synchronising require- ments each bit is more complex than that for a separate-clock pulse system, and consists of an initial period at logic 1; a following period at logic 0 for synchronisation; a period defining the bit value, i.e. 0 or 1; and a final stop period at logic 1.
The formation of a complete D2B message is given in Fig. 22.7.
First is a start period at logic 0, then a mode indication to define which of the three speeds is to be used. A 12-bit code to identify the master is now sent, followed by a parity bit for truth checking. The next 12-bit word addresses the intended slave, which will be a piece of AV equipment, a lighting circuit or perhaps even an oven. Parity and acknowledge bits follow, then a 4-bit control signal to define direction of transfer and type of message. Finally comes the control data itself, an 8-bit word to convey a possible 256 different commands, followed by a ‘sign-off’ set of continuity, parity and acknowledge bits.
In practice the D2B bus protocol has seldom been used, and SCART pins 10 and 12 have been appropriated by equipment manufacturers for other purposes such as satellite-dish positioning drive or data, and power feed lines. Unless one is sure of the control function at both ends (if any), then, it is best to use SCART leads whose pins 10 and 12 are not connected: this avoids the risk of malfunction and damage.