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Both the local and remote UARTs must be configured for the same communications parameters. The local and remote must share a common ground, so all serial cables include at least one ground conductor. The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). 12 volts and outputs logic level (0 or 5 volt) signals to the UART circuitry. RS-485 signals are used in a wide range of computer and automation systems. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. When the keyword name is received by the Silence() routine running in the slave, the slave PDQ Board executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). Now there was certain instances I used STP wire for rs485 because I had a lot of devices connected in a long run chain (DMX512 lighting fixtures). Although the devices would share the same network, communications would only be understandable by members of the same group.

Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. The serial output at the UART idles at the logic high (mark) level. High (mark) parity means that the parity bit is always logic 1 at the UART, and low (space) parity means that the parity bit is always logic 0 at the UART. Even parity means that the bits sum to an even number, and odd parity means that the bits sum to an odd number. The master and slave could even exchange ascii QED-Forth operating system commands. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register.

A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. The above parity settings will also determine how incoming data is interpreted (whether the most significant bit is considered a parity bit or part of the data being transmitted, and how many bits total to expect in each byte). While running this program, the parity settings of Mosaic Terminal may be adjusted, and in each case the message that matches current settings will appear clearly while the other messages will appear garbled. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. If PT is cleared, then all transmitted bytes with a parity bit will have an even number of total '1' bits. However, verifying correct parity of bytes received with a parity bit is currently not supported. Once the bytes have been exchanged, the master may write a new byte to initiate another byte exchange. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus.

The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii name of this particular slave). In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS. The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. The words port and channel are used interchangeably to refer to a serial communications link. Owing to hardware constraints, if modem handshaking is needed on UART channel 1, then channel 1 must be configured for RS232, and channel 2 cannot be configured for RS232 communications. The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them.

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