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It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. When the keyword name is received by the Silence() routine running in the slave, the slave QScreen Controller executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. The termination also includes pull up and pull down resistors to establish bias for each data wire for the case when the lines are not being driven by any device. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides.

RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. 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. If the /SS input to a slave is inactive (high), the slave ignores the SCK input, does not transmit or receive data, and keeps its MISO output in a high-impedance state so that it does not interfere with the SPI bus. The /SS input in turn determines whether the slave responds to the SCK input, as described in a previous section. Thus, the master 68HC11 has only one input, MISO, which is the slave QVGA Controller’s only output. The next section describes the registers that configure and control the QVGA Controller’s SPI. Communications capability is essential for many instrument control applications. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data.

Hardware is interfaced to the SPI via four PORTD pins named /SS, SCK, MOSI, and MISO brought out to pins 11 through 14 on the Digital I/O connector (see Appendix A). The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. RS-485, like RS-422, can be made full-duplex by using four wires. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. Function prototypes for this function and other versatile serial I/O routines are defined in the COMM.H header file, and are described in detail in the Control-C Glossary. A serial communications cable is also supplied with QScreen Starter Kits. 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. 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. From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications.

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. In this case, what is rs485 cable cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables. To reduce the effect of interference from external electrical signals the RS485 interface uses a differential signal over a "twisted pair" cable. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. The data exchange format may be a line of ascii text.