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 pinout of the PDQ Board’s Communications Header (H2), Docking Panel’s Communications Header (H1), and the Docking Panel’s Communications DB-9 Connectors are shown in the following tables. The following table shows the connection diagram for a standard 9-pin serial cable. Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections. Resistive termination - If the PDQ Board is at the end of the RS485 cable you can terminate the cable by installing jumper caps at both jumper locations, "Term" and "RTerm". For Serial1 RS485 operation: Install the jumper shunt onto "1 485En" (J4). The RS485 interface can use RJ45 jacks for CAT-5 cable or a terminal block. No termination - If the PDQ Board is not an end device, you should not terminate that cable. 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. 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. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector. 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. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. RS232 is by far the most common serial protocol, and is the default protocol for both of the PDQ Board’s serial ports. 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.

Chassis and signal grounds are connected together to the digital ground (DGND) signal. 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). Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. Cables to expand or extend the RS485 system. 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. RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. RS232’s greatest benefit is its universality; most personal computers can use this protocol to send and receive serial data. 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.

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 main difference between them is in the hardware. The PDQ Board, however, does not implement hardware handshaking. 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). 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 what I’ve read, the modbus is a protocol for communication between a computer and an RS485 device, if this is true, I shouldn’t need a modbus lib for this application. RS485 is used as the physical layer for many standards and proprietary automation protocols to implement control systems, most commonly Modbus. Overall, RS485 cables are known for their robustness, reliability, and versatility, making them a popular choice for industrial automation, building control systems, and other applications requiring stable and secure communication over extended distances.