I am software enginer and working with board with rs485 rs485 transceiver is 3.3V ST3485E and is connected with PLC. When I send data from my board to PLC, voltage level is 3.3V but PLC send 5V data. PLC recognize 3.3V input data in 1200bps but not 9600 bps. when PLC recognize 1200bps signal it sends reponse which my board can recognize. and garbage data that PLC doesn't send is continuously received by rs485 transceiver. I wonder if this is correct situation. rs485 Vdd is 3.3V and receives
5V signal and it can recognize 1200bps signal but not 9600. and continuous garbage data cause interrupts. I guess we should use 5V transceiver but h/w guy says this is correct situation and when they connect 2 GND lines, pheonomenon will be corrected. Please anyone help me solve this problem
RS-485 is a differential protocol, so as long as the receiver sees at least a +/-200 mV difference between the A and B lines, it should detect the characters correctly.
However, there is the common mode voltage range issue, but if the 3.3 V device claims RS-485 compatibility, +5 V is definitely within the common mode voltage range.
However, the 3.3 V transmitter voltage swing is smaller, so you do not tolerate as much voltage drop on long lines as with 5 V devices. However, since the receiver requirement is only +/-200 mV, there should not be problems with short cables.
When neither transceiver is driving the bus, it floats in three-state and the receiver may pick up random noise. Use the "fail-safe" termination specified in the RS-485 standard. Put a resistor equivalent to the characteristic impedance of the cable (typically
100-120 ohms) to both extreme ends of the bus and bias the bus into "Mark" state by pulling the other line to Vcc and the other to Gnd at one point of the bus with 1-10 kohm resistors. This will create a passive "Mark" ("1") state, when no transceivers are active.
You have other problems with 9600 bit/s (data overrun?), but the continuous garbage is due to the missing "fail-safe" termination.
The 3.3 V transmitter might not be able to run 115kbit/s using thin wires, but apart from that, this should not be a problem.
You either have to connect the GND lines or run with ust the A and B lines with proper termination (and "Mark" biasing at both ends, if the devices are truly floating).
There should be a ground connection to keep the common mode voltage within reason. Fix that first. There's no inherent problem with the different supply voltages.
Also beware that the definition of A and B circuits, as defined in the RS-485 spec, is inverted with respect to the "standard" SN75176-type transceiver. No, really. It's a long-standing fuckup.
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