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The balanced receiver sensitivity treshold is high enough that in many scenarios, ingress EMI is not an issue (some point out that RS485 is more immune than CAN). RS-485 and RS-422 can interoperate with certain restrictions. Since the majority of RS-485 applications are driven by a UART, the data rate is usually below 100kbit. In this case, POTS telephone wire should work for quite a long line. In the aforementioned customer trouble case, there were actually several problems combined. Surprisingly, there wasn't much common-mode wobble (reflections). By now the signal was really nice and clean, with only small common-mode glitches on the edges (insignificant). 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). The sensitivity treshold and hysteresis in the receiver certainly play a role. Since a receiver may have a loading of less than one, the actual number of receivers that can be connected depend on the unit load rating of the receivers, as well as the wire, bit rate, stub lengths, biasing and termination of the network. Since the RS-485 driver is "made passive" (disconnected from the wires) when not transmitting, and the other end of the cable can then drive the network, a termination resistor should be added to both ends.

It doesn't even have to be a master node - the biasing node can just as well be a protocol slave, or the biasing resistors can be added someplace halfway between the nodes on a bare transmission line. I added a proper second terminator - to no avail. The third installation uses the same two computers as the second example, but they are separated by several thousand feet of wire, and one of the computers is sitting next to an arc furnace that draws several thousand amps when operating. The graph at right, which is often shown in these app-notes, shows a limit of 1200 meters/DC at one end and 14 meters/10 Mbit at the other. All of the application guides and data sheets that say RS-485 has a limit of 1200 meters or 10Mbit are flat out wrong. At the other end of the cable is (typically) the same thing -- the connectors, the RS-485 interface IC, and a UART inside a microcontroller. Alas, a long 3-conductor cable intended for RS-232 can not be switched to full-duplex RS-485, which requires 5 conductors. Anyway - nodes with a properly isolated RS485 port can be attached to the shield ground directly, without this 100Ω resistor, and such direct ground connection may prevent some common-mode wobbling of the pair against the shield.

The local 232 port will likely remain un-isolated from the converter's MCU, if there is any. But this is only true if there is no ringing or noise on the line that switches the receiver back to a 0 at the end of the stop bit. The meter did not have a ref.GND terminal, nothing to connect there. 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. A RS-485 interface will usually use pins 7 and 8 for the two data lines, since they comprise a twisted pair. TODO: add connectors and pins from manufacturers such as NI, B&B, ADAM, etc. Volunteers? Devices with the same connector but from different manufacturers are probably not directly inter-connectable. 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.

This problem can also be fixed by adding bias resistors to force the line to idle condition when a driver is not connected, but this has to be done on a network basis, not a device basis. Modbus devices communicate using a Master-Slave technique where only one device (the Master) can initiate transactions (AKA queries). If your cabling is not the super-short no-brainer anymore, i.e. the length transposed to round trip time gets close to maybe 10% of the data bit length, you should get a proper cable in the first place (with the right impedance and a low RF attenuation), so that you can terminate the transmission line properly, without hampering the tranceivers' DC operating conditions. 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. 12V. A driver in the high impedence (off) state is able to remain in this state over the common mode range, whether power is applied or not. So I let the SCADA software crank away, to show me some traffic, and tweaked the scope a bit - looked at the fuzz around bit edges, possible interference, differential mode, RS485 standard common mode.