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Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. 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. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. With careful design, many peripherals can communicate via the SPI, and powerful multi-processor systems can be linked using this high speed bus. There are several protocols that govern the format of exchanged data, with the RS232 protocol used primarily by personal computers, and the RS485 protocol used in industrial control systems. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller.

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 QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. Data translation between different machines can be performed with ease, and applications that communicate via the primary serial port can be debugged using the secondary channel. Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. 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. The terminal’s serial receiver chip re-inverts the signal to its positive sense. Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1.

The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. Newer protocols include the full duplex RS422 and the half duplex RS485 protocols, each of which drives differential 0 to 5 volt signals on the serial cable. It is a half duplex protocol, meaning that only one party at a time may transmit data. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. A serial communications cable is also supplied with QScreen Starter Kits. Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections.

In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. Care must be taken when changing an RS485 node from transmit mode to receive mode, or visa versa. The master can instruct a single slave to go into transmit mode, and then the master can put itself into receive mode, thereby allowing the master to retrieve data from the slave. In RS485 mode, the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers as explained in the UART Wildcard Hardware: RS485 Jumpers section below. It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. The master and slave can then exchange data. This ability to exchange messages means that the SPI is capable of full duplex communication. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications.

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