Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). Large phone cables have solid with white stripes, and solid. 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. You might make it work anyway, with a bit of stuffing at the protocol level, or it might "just work" with sparse communication and a receiver that interprets 0V differential as idle. The below image shows the transmitter and receiver circuit diagram for Arduino's long-distance wired communication. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. One uses the data lines for 10/100 Ethernet, so that there are still 2 unused pairs in that cable. But you can still get some induced noise from one to the other, partly because the twists aren't perfect and they're not shielded from each other.

The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. RS-485 does not specify any connector or pinout. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Care must be taken when using A/B naming. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. In this case, cable connections may be made to Serial 2 at pins 4 and 10 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. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector. 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 only way this would not work is if the secondary device only takes "write only to device" transmissions with no possibility of any acknowledgement or return result. For these secondary devices you have to implement your periodic heart beat check in some device specific way. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. The switch version requires a DC average of 0V on the communication lines, which Ethernet satisfies but RS485 doesn't. 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). RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for full duplex RS232 communications: a transmit wire, a receive wire, and a common ground.

Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. So I would not run low level MCU GPIO level signals over the cable wires. RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. There is also the GSM version; especially useful when only one sensor is needed in a remote location where other types of transmission cannot be used. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors.

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