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The terminators consist of just a single resistor, i.e. the conventional way - just a diff-mode termination, no common-mode termination. The only way to make them talk, was to connect exactly one terminator (plain 120Ω resistor) - not two. Basic termination networks, such as standard parallel termination networks, RS485 standard typically employ a single resistor. The standard 120-Ω value for termination resistors corresponds to the differential-mode characteristic impedance of the twisted-pair bus wires. The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable. Other receivers on the network that do not have this shifted receive threshold may require that external bias resistors added to the network. The morale for me is that, for short sections of cabling, even if you don't have a 120Ω cable, you should still use 120Ω terminators, because it's more important to please the active transceivers, than to correctly terminate the transmission line. RS485 is a standard defining the electrical characteristics of drivers and receivers for use in serial communications systems, widely used in industrial field. This standard is for a network of a single balanced pair of wires. This means there will be times when no driver is connected to the wires.

If you have to know that it will work before you pull the cable, then get the required length of the cable and move the two devices next to each other, connect them and see if they can talk. Earth isolation is expensive, may require purchasing of additional isolated PSU's (where you already have a grounded PSU from the vendor), or purchasing of addon isolated bus transceivers (when you already have a non-isolated port on your device, chosen for other reasons). Other standards may define the connectors and signals on the pins of the connector. In addition to the 21 circuits, the shield may be connected to pin 1, pins 9 and 10 are "Reserved for Testing", and pin 11 is "unassigned". This creates a fail-safe bias voltage that causes the negative terminal to have a lower voltage than the positive terminal and the output of the comparator to be in a known high state when applying a 0-V differential voltage to the A and B pins.

My explanation is that the RS485 bus drivers' output impedance is optimized (matched) for the typical 120Ω transmission line (the driver's output impedance should actually be half that value, because the driver feeds two sections of the TML in parallel), and if a lower impedance transmission line is attached to the driver, the drivers' output sags accordingly. Next, I mapped their transmission line. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s. 12V PSU rail to pin 9 in the COM port (which can otherwise be jumpered for RS232/422/485), instead of the standard function of a "ring indicator" input. Circuit AB is on pin 7 and is described as "Signal Common". The RS-485 circuit common is connected to earth ground in both PCs. 5V PSU, and referenced to the site's overall protective earth. The "reference ground weakening" resistors are a tradeoff measure - they allow you to use a shielded cable, to have the shield ground somewhat referenced to the RS485 transceivers, yet avoiding a hard ground loop with the mains protective earth.

It finds extensive use in applications such as factory and building automation, motor control, and other field bus applications. So there I was, with the 120Ω terminators back in place, and the bus didn't quite work, despite the nice-looking scope traces. There was some "interference" though - and it was common mode. This appendix will attempt to explain what RS-232, RS-422, and RS-485 are and are not; then discuss one of the more common implementations of RS-422 and RS-485, asynchronous start-stop ASCII communication with a UART. None of these protocols are part of the RS-485 standard, and implementation is up to the engineer writing the software. From a software point of view, full-duplex RS-485 looks very similar to RS-232. 125 mV. The small voltage gradient is intended to improve noise immunity of the idle bus, as 200 mV differential is the sensitivity treshold of the bus receivers (upper toggling point of their built-in hysteresis).