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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. 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. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. 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. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors. Once the bytes have been exchanged, the master may write a new byte to initiate another byte exchange.

The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte. 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. A modem (modulator/demodulator) provides a way of encoding digital data as a set of audio signals that can be sent over a telephone line. RS-485 signals are used in a wide range of computer and automation systems. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. 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. 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.

The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. 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 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. 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. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications.

The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. SPIE is a local interrupt mask that allows an interrupt to be recognized when an SPI data transfer has completed, or if a write collision or mode fault is detected. If your application requires use of the secondary serial port as well as other interrupt routines, rs485 cable the key is to keep the interrupt service routines short and fast. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. The QScreen Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector). The terminal’s serial receiver chip re-inverts the signal to its positive sense.