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For devices where you have to consider the devices at the far end of your cables to be unable to be modified, either hardware or software wise, you have to get clever regarding how you determine if the device is connected. The device ships with an Ethernet RS-485 communication interface as well as a RS-485 to USB cable, which is said to be able to communicate with a PC using proprietary Windows based software (and driver, I think). There is a huge benefit of the RS-485 electrical layer protocol that allows for long distance communications in the presence of multiple volts of common mode voltage differences between the two ends. If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable. The end resistance must be used only at the ends of the main cable. This distance does not include the branches (which must nevertheless be short).

The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. The RS485 data direction of Serial1 and Serial2 are controlled by the PJ0 and PJ1 processor pins, respectively. It is the ideal solution for connecting industrial devices such as laboratory testing and measuring equipment, monitoring devices, material handling equipment, and many more devices where mission critical applications are required. The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus.

We will pull the enable pin high to put the MAX485 module in receiver mode. 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 connection between two or more elements (drivers and receivers) should be considered a transmission line if the rise and/or fall time is less than half the time for the signal to travel from the transmitter to the receiver. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. 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. But You stated you are going to install this at a commercial setting, so it always better to use the worst case environment situations, and use shielded cable.

Is it necessary to do the ethernet cable splicing as indicated in the above link, or should I simply be able to use the provided USB cable? Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections. Communications capability is essential for many instrument control applications. 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. 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 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).

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