All of the RS232 signals start with the / (slash, pronounced not) character to indicate that the signals on the serial cable are logically inverted. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. The Serial 1 and Serial2 ports can be configured for either RS-232 or RS-485 communications at standard baud rates up to 115200 bits per second. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground. Likewise, the terminal’s transmit signal /TxD is connected to the PDQ Board’s receive signal /RxD1. 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).

If this limitation is not a problem, you can reverse the roles of the Serial1 and Serial2 ports, because they have identical communications capabilities. With this boards, a CBL03 lines with RJ11 connectors can branch out to support an RS485 line consisting of three discrete wires. In this case it is possible to connect the main inlet and outlet terminal directly to the terminals of an instrument without creating a branch. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the PDQ Board) is ready to accept data. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. RS232 uses inverse logic; that is, a positive bit at the HCS12 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. 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.

RS485 is another protocol supported by the primary serial port on the PDQ Board. Having a second serial port is also handy for system debugging. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel. Be sure to account for these effects when designing your application. They translate the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the operating system or by your application program. After a factory cleanup, Serial1 is the default serial port for program development and downloading. The default serial routines used to download programs to the operating system assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. The advantage of using Serial1 for RS485 is that the Serial1 RS485 signals are also available on the Docking Panel, while the Serial2 RS485 signals are available only on the PDQ Board’s Serial Communications Header. These pins are normally used for the DTR, data terminal ready and DSR, data set ready signals.

By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the PDQ Board is always ready to send and receive data. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. Rather, it relies on software handshaking via transmission of XON/XOFF characters (ascii 0x11 and 0x13, respectively) to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. After the slave transmission is complete, the slave puts itself back into receive mode so that the master can transmit additional commands. 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 PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. These protocols are summarized on this page, but for more information regarding their data formats and their use for simplex or multi-drop serial lines, consult Understanding Serial Communications (but keep in mind that that page is directed to the use of the UART Wildcard, so it uses different driver functions).

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