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). 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). From the PDQ Board’s point of view, these three signals (/TxD, /RxD, and ground) are the only connections required to perform serial communications. 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 PDQ Board, however, does not implement hardware handshaking. The transmit and receive data signals carry the messages being communicated between the PDQ Board and the PC or terminal. 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.

The mating 10-pin connectors that join the H6 header of the PDQ Board to the H4 header of the Docking Panel are typically not accessed directly, and are not discussed in detail here. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the PDQ Board brings out its serial ports to two female 9-pin D-Type connectors on the Docking Panel. Thus in the table, /RTS1 is connected to /CTS1, and /DSR1 is connected to /DTR1 and /DCD1 onboard the PDQ Board using zero ohm shorting resistors. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). Two devices are at the ends of the cable, while others are connected somewhere in between.

The end devices are responsible for terminating the cable so that there are no reflections from the cable ends. RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, rs485 cable although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. Please contact Mosaic Industries if you need this custom configuration. Contact Mosaic if you require RS485 signals to be routed to the DB-9 Connector. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. These 9-pin standard DB-9 serial connectors are located on the back of the Docking Panel. The PDQ Board’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 symmetrical pin layout of the RJ11 sockets on the Unitronics PLCs allows an easy way of interconnecting the PLCs with both RS232 and RS485 communications. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds.

Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. In that case you may terminate the lines with a series RC network comprising a 0.1 μF capacitor in series with a 120 Ω resistor. In this case, cable connections may be made to Serial 2 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, or the Docking Panel’s Serial2 DB-9 Connector. In this case, cable connections may be made to Serial 1 at pins 5 and 6 of the PDQ Board’s 10-pin Serial Header , or pins 5 and 6 of the Docking Panel’s 10-pin right-angle Serial Header. No termination - If the PDQ Board is not an end device, you should not terminate that cable. The other end of the cable should be terminated similarly. And the results before was a degraded signal that the devices near the end would interpret the signal incorrectly.