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Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second. The BAUD routine described at the start of this subsection configures the baud rate of each of the Serial1 and Serial2 channels. The Serial1 and Serial2 ports are is supported by the HCS12's dual on-chip hardware UARTs, and do not require interrupts to work properly. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. If this limitation is not a problem, you can reverse the roles of the Serial1 and Serial2 ports, because they have identical communications capabilities. The symmetrical pin layout of the RJ11 sockets on the Unitronics PLCs allows an easy way of interconnecting the PLCs with both RS232 and RS485 communications. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. The PDQ Board's two serial ports support limited use of generating a parity bit. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the PDQ Board brings out its serial ports to two female 9-pin D-Type connectors on the Docking Panel.

We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in the above table. The UART Wildcard supports any baud rate produced by the above formula. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). Each of the two channels on the UART Wildcard implements two 16-character FIFOs, one for outgoing characters and one for incoming characters. When PE is set (equal to one), the most-significant bit in each byte transmitted will be a parity bit that is either set or cleared by the serial port automatically in order to achieve even or odd parity. If PT is cleared, then all transmitted bytes with a parity bit will have an even number of total '1' bits. If PT is set, all transmitted bytes with a parity bit will have an odd number of total '1' bits. In either of these cases, a source of noise that caused one bit to be received incorrectly would invalidate the received byte, since the total number of '1' bits would be odd rather than even.

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. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. You can use one or both of the PDQ Board’s RS485 links to create such a multi-drop serial network. This page describes the serial ports and how to use them for instrument control and automation applications. Data translation between different machines can be performed with ease, and applications that communicate via the one serial port can be debugged using the other serial channel. RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end.

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. If your computer does not have an RS232 serial port, rs485 cable low cost USB-to-RS232 serial cables are available; contact Mosaic Industries for details. Contact Mosaic if you require RS485 signals to be routed to the DB-9 Connector. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector. A jumper labeled "2 485En" (J7) enables RS485 operation on the Serial2 port if the jumper cap is installed, and configures Serial2 for RS232 operation if the jumper cap is not installed. The USE.SERIAL2 command means that the operating system’s terminal interface now communicates via Serial2. The DECIMAL command makes sure that numbers are interpreted in the decimal base. 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. The master is in charge of designating which receiver is on at any one time. RS232 could only handle one.