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This computer had limited functionality with a keyboard and monitor as the primary peripherals. I ordered my steps by "layers of functionality" and also by convenience (try the simple things first, leave time-consuming stuff for the last resort). 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. Typical UART communications with an RS-422 device, uses both a transmitter and receiver on a network of two balanced pairs of wires for a total of four wires. A higher value termination resistor will reduce the DC losses associated with extreme line lengths, allowing for much longer line lengths at the cost of ringing on the wires. The exact voltages will depend on the driver, the loading, biasing, termination, and any shift in ground potential between the driver and receiver.

It shows a diagram of the driver and receiver with two wires connecting them, and a third point "C" that is called a common. In general, many current applications for RS232 use only the electrical standard (3-wires, TDX, RXD, Common) and connector pin-out. This in turn means that the drivers' heat dissipation increases significantly (more current and a greater voltage drop across the output transistors), which is certainly a safety risk. This means that in a master/slave configuration, a master can talk to multiple slaves, all of the slaves can talk back to the master, and every device on the network can hear every other device. In their wiring, the PC would talk just fine if only the I/O module was connected, or if only the meter was connected. If both the meter and the I/O module got connected, the data reading loop would stop, and only give another retry every 20 seconds (in vain).

The customer had my culprit PC connected to just two devices (RS485 slaves): an I/O module (not our hardware) and a meter device - for a total of three nodes, from three different manufacturers. You can find out more about the differences between RS485 and RS232 here. 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. Next, RS485 standard I mapped their transmission line. Articles, application notes, even data sheets from semiconductor manufacturers discuss both the data rate and line length limits in RS-485. Many other IC manufacturers make a drop-in replacement for this IC, and many of them (such as the MAX483) use the same labeling. Now being used commonly in the pro audio industry to control digital audio and signal processors such as the DBX driverack and other manufacturers equivalent products. By now the signal was really nice and clean, with only small common-mode glitches on the edges (insignificant). DC bias), whereas responses from the meter were centered around 0V, so that both pins in the signal pair went significantly below 0 at times.

This standard specifies the electrical characteristics of a single transmitter and up to ten receivers on a single pair of wires. Each pair has two wires to which a driver and one more receivers are connected. But it does offer some more insight in the implementation that i suspect has ether been neglected or ignored by some big brands building RS485 devices. But: If you control all nodes on the network RS485 can be pretty darn good. Define any network biasing I.e: What is the "correct" idle voltage on the bus? The bus did indeed swiftly return to 0V when idle. Two-way resistor termination: Works best on a linear bus, with transmitters anywhere along the bus. The only way to make them talk, was to connect exactly one terminator (plain 120Ω resistor) - not two. Which ultimately prompted me to add the two 560Ω biasing resistors. This is very convenient for the biasing function. 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.