Consult their Glossary Entries for Details

Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). 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). It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. 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. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. The USB to RS485 Converter is a cost-effective module for transfer serial data over USB. The USB to RS485 Converter is designed to make serial port expansion quick and simple. 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.

We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. Similarly, when establishing the transmit mode, the application software is responsible for ensuring that the RS485 receiver is not disabled until all expected characters have been received. 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. If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte.

Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device. 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. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus. Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B).

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. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. 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. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable.

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