Setting SPE (SPI enable) to 1 turns on the SPI system. If your application requires communicating with a device that expects to receive a parity bit, the generation of a parity bit and selection of even or odd parity, and whether there are seven or eight data bits in each byte, is performed by setting or clearing bits in the configuration registers SCI0CR1 for Serial1 and SCI1CR1 for Serial2. It requires minimal wiring and can share the wiring among several seats, reducing weight. The remaining inactive slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. The master and slave could even exchange ascii QED-Forth commands. At any given time, only the master and a single "active" slave communicate. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged.

Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs).

RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Because all of the serial I/O routines on the PDQ Board are revectorable, it is very easy to change the serial port in use without modifying any high level code. If the clock idles in the high state, the leading edge of the clock is a falling edge. The CPOL and CPHA bits configure the synchronous clock polarity and phase and specify when valid data is present on the MISO and MOSI data lines. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.

In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. These are used in programmable logic controllers and on factory floors. After a factory cleanup, Serial1 is the default serial port for program development and downloading. These 9-pin standard DB-9 serial connectors are located on the back of the Docking Panel. In this case, cable connections may be made to Serial 1 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, rs485 cable or the Docking Panel’s Serial1 DB-9 Connector. The pinout of the PDQ Board’s Communications Header (H2), Docking Panel’s Communications Header (H1), and the Docking Panel’s Communications DB-9 Connectors are shown in the following tables. Two RS485 transceivers are present on the PDQ Board, one for each channel. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel.