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RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. This 6 ft long cable comes with a USB connector on one end and the red and yellow tipped plugs and does not require a driver if used with Windows 7 or later versions. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. The RS485 data direction of Serial1 and Serial2 are controlled by the PJ0 and PJ1 processor pins, respectively. The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. 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.

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. The primary and secondary serial communications ports are accessible through the PDQ Board's 10 pin, dual row Communications Header (H2) and through the Docking Panel's 10 pin, right-angle, dual row Communications Header (H1) and individual DB-9 Serial 1 and Serial 2 connectors. The Serial1 and Serial2 ports are is supported by the HCS12's dual on-chip hardware UARTs, and do not require interrupts to work properly. The Serial ports are implemented by the dual on-chip hardware UARTs (Universal Asynchronous Receiver/Transmitters) on the Freescale 9S12 (HCS12) microcontroller. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. After a factory cleanup, Serial1 is the default serial port for program development and downloading. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. So long as the error between the actual baud rate and that specified is less than 1.5% (or the error between transmitter and receiver is less than 3%) there should be no communication errors.

The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs). Each UART implements transmit and receive FIFO buffers to reduce the required frequency of service by the host processor. A break sequence forces the serial output to a logic low (space) at the UART. 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). EIA-485 only specifies electrical characteristics of the driver and the receiver.

We will pull the enable pin high to put the MAX485 module in receiver mode. Here again, we start with including the standard library for driving the LCD and declare the D8 pin of the Arduino Nano as an output pin which we will later use to declare the MAX485 Module as a transmitter or Receiver. 12 V and -7 volts will allow correct operation of the receiver. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven.

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