Hello, I have small project do control Schneider VFD Altivar 320 with 750-8102 over modbus RTU.
I have connected device to RS485 and when I read holding registers I get error CRC checksum fail or invalid data response. Comm setting are correct. Devices are connected with 0.5 meter cable without termination. I have wired rs485 pin 3 to RJ45 connector on Altivar VFD to D1 and pin 8 to D0.
Is it correct wiring or I need to jump wires on rs485 connector? Can anything else be problem ?
I haven’t checked your pin assignment, but I would try with termination including the pullup / pulldown resistors. I’ve had problems with rs485 where my equipment have recieved the wrong data because of the small electrical disturbances when switching between sending and listening. The easiest way to add the right resistors is to use a contact for profibus like 750-960.
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Hello,
i have purchased Wago profibus connector.
Can you explain proper termination insert on connector?
It’s easy. You just activate the termination with the little switch on the contact. then you get ok resistors between A & B plus resistors to 0V and 5V as well.
in built-in RS-485 you need to terminate it in controller side. 120-150 Ohm between 3 and 8 pin.
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Hello @WagoDamRud: If we have a Modbus Device with external Power, such as 12-24Volts (V+, V-, L+ (A) ,L-(B) ), which is the preferred wiring of the Wago PFC
| Device | PFC |
|---|---|
| A | 3 |
| B | 8 |
Thank u for your Reply, but what happened with 5V, 0V from the Wago Side?
For what it may be worth, the biasing schematic shown earlier in the thread is identical to that in the PFC 200 manual. I too had no success with Modbus RTU until creating the 120 ohm line termination and 640 ohm V±V- biasing configuration within the RS-485 hood. Now, any data rate with any RTU sensor is without issue. Line lengths to 300 meters have been proven over nothing more than twisted 18 AWG control cable. Far from the proper cable. It’s a robust interface provided you find the right combination for the bias circuit.
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