Modbus on AB microLogix 1500

[fbliman]

Hi,

I am trying to connect a micrologix 1500 (1764-LRP) to a Hitachi SJ700-2 inverter.
To connect them i am using ch1 in the PLC (RS-232), a rs-232 to 485 converter, and then i connect it to the 485 port in the inverter.
The PLC is used as master ande the inverter as slave.
I have to read and write some registers. I tried writing and i could, but i didn´t get the ack back. The same happened when i tried to read registers, i dont´t get the data from the inverter.
To do this i am using the msg instruction on rslogix 500.
The parameters i used are:

MB command: 06 write holdind registers
data table address: N10:0
size in elements:1
data:16 bits

message timeout: 2
MB data address: 1
Local bridge addr: 1

the error i get is: message timed out in local processor (37h)

Does someone knows what may be happening???

Thanks!!

 

[Ken Roach]

The problem could be in the RS232/485 converter, or in the timing and addressing of the message.

Are you certain that the inverter is using Modbus Slave address 1 ? Are you sure it is using the same data rate and framing (data bits, parity, stop bits) as the MicroLogix ?

Post more details about the RS-232/485 converter and how you have it connected to the MicroLogix and to the inverter.

With "2-wire" RS485, always try switching the Data A and Data B wires.

Remember also that "2-wire" RS-485 does need a third wire for the common voltage reference. Often earth ground is used with nonisolated devices. The connection provided by the inverter and by the 232/485 converter will determine what you have to connect.