These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. The distances these signals are carried is greater due to differential signals. Overall, RS485 cables are known for their robustness, reliability, and versatility, making them a popular choice for industrial automation, building control systems, and other applications requiring stable and secure communication over extended distances. Communications capability is essential for many instrument control applications. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. This section describes the driver routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. One to two logic-high stop bits mark the end of a character. The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. A UART is a Universal Asynchronous Receiver/Transmitter that converts parallel data from the host processor (any Mosaic controller) into a serial data stream.

For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). This automatically activates the SCK clock which synchronously transmits the data. The serial interface is asynchronous, meaning that there is no clock transmitted along with the data. Bauddesired is an unsigned integer from 1 to 56000, 500000 is the frequency of the UART's internal clock and Round(500000/Bauddesired) is an internal divisor (rounded to the nearest integer).

The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. Baud to specify the baud rate. The UART Wildcard supports any baud rate produced by the above formula. Both the local and remote UARTs must be configured for the same baud rate. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. RS232 is by far the most common protocol. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. After the slave transmission is complete, the slave puts itself back into receive mode so that the master can transmit additional commands. If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. 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. 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).

A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. In other words, each local UART on the wildcard can both send data to and receive data from a remote UART on the other end of a connecting serial cable. 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. In the most common multi-drop RS485 protocol, one computer is designated as a master and the rest of the computers or devices on the serial bus are designated as slaves. Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. The ideal RS485 network requires a dedicated wire for ground. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. It gets away most of the time with this configuration, with the shield draining noise and providing the ground reference at the same time.

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