Problem about Modbus communication between AC800M PM856 COM3 and RS485 2 WIRE
I am trying to build up Modbus RTU connection with a RS485 device.
i connect TK212 to COM3 on PM856, connect the other side to Phoenxi Psm-ME-RS232/RS485(ORDER NO 2744416).
I cannot get stable valid from the MBConnect output.
I have tried to swap the 2 wires of RS485, when the wire touched the pin , the valid would be true for some seconds then disapeared
I tried to reduce baudrate from 9.6 to 4.8, it didn't work.
It is possible to get data from the 2 wire RS485 by Modbus Poll without converter board.
9.6enumKBIT/S noneenum 8enum 1enum Noneenum 0dintmsec0255
Master enum 3 dint 1 247 4000 dint msec 0 8000 2000 dint msec 500 6000 3 dint 1 6 100 dint 0 65535 101 dint 0 65535
the setting of converter board is
1) slave address should be same
2)baud rate and parity bit should be same.
For modbus rtu communication AC 800M supports only RS232 protocol for thatRS485/RS232 converter required.
If you are using the RS 485 2 wire connection directly connected to the D+ &D- at the converter and if you are using 4 wire connection, combine T+ ,T- and R+,R- . after that combination of T+ and T- connected to D+ and R+ and R- is connected to D-.straight to straight connection.
From output of the converter, in Ethernet cable 2,3 pin (2:receiver and 3: transmitter & 5 is ground) ground pin should be remains.
From my knowledge, you need to prepare your own cable for COM3, TK212 might not work.
Use only pins 3,4 and 6 from COM3 and connect them to corresponding pins of RS232/RS485 converter (TD to RX and RD to TX - crossed).
Look RJ45 pin layout for COM3 below. 9,6k should be OK.
The other useful hint - set Poll Time = 0 for Modbus protocol.
Last - Valid on Connect block is not enough.
You have to receive output Ndr=1 for MBRead block. Use periodic pulses on Req input of MBRead block.