Serial communication problem

[Menios]

I am trying to communicate with a power analyser,
using Lookout Modbus object. The analyser has an RS485
interface and for this reason I use an RS485/RS232 adapter. When I am trying to establish communication, I get the message : No communication response from Modbus secondary. I tried to increase Receive Time Out in Lookout and I have checked all the communication settings (baudrate, databits, stopbits,parity, Modbus address). I would appreciate if anyone could suggest something about my problem.

[Patrick R.]

If you analyser has a RS485 interface, check to see if it uses full-duplex(4 wire) or half-duplex(2 wire) mode, making sure that the 2 wire communication of RS232 is sufficient.

Determine if the analyser communicates with the RTU protocol or the ASCII protocol. Lookout tries the RTU protocol first by default and then the ASCII so the Receive timeout may play a part in not receiving a response from the device.

Next, try to communicate with the device via Hyperterminal. If you reference the COM port and write a basic command to the device, you should see a response. This will ensure that the communication through the adapter is working properly.

I would also try increasing the Receive Gap setting from the default of 20 Bytes to 40. This setting is found in
the Serial port settings of Lookout. You can access these settings by going to the "Options" menu. If there is a slower baud rate being used, or if the device is slightly slower in responding, the receive gap may prevent data from being misinterpreted.

I hope that this information is helpful. Have a good day.

[Menios]

Thank you very much for your suggestions. I checked my analyser and it has a 2wire rs485 interface and supports Modbus RTU protocol.Additionaly I tried to increase the Receive Gap, but with no result. I didn't try to communicate via Hyperterminal but I enabled the COM monitoring of Lookout and as I saw there was no response from my analyser(additionally only the converter led that indicates data flow from the PC is flashing the other one is always off). The command sent from Lookout seemed to be correct (analyser address,command code, register address, number of bytes and CRC).
I also connected a device that operates as a Modbus RTU master to this adapter and the communication with Lookout (using the Modbus Slave object) works fine.
Is it possible that th
is adapter is not appropriate for the comunication of my analyser with Lookout, that is it can only work when a Modbus master device is connected in the RS485 side?

[Patrick R.]

From what you have described, it would appear to me that something in the adapter or in the 485 connections is the problem. The first thing I would suggest is to test the adapter to verify that it is set up properly. If the 485 output is 2-wire, you can test the adapter by connecting these two wires together. After doing this, you can use hyperterminal to see if you get an echo of the characters that you type. This is known as a loopback test in hyperterminal. Also, you could open your lookout process and log the communications again. You will probably get an alarm that says "Gargled communication from Modbus" but when viewing the file, you should see the same command come back that was sent out.

If the adapter works ok, I would verify that yo
u are making the connections properly to the analyzer. The Rx line from the adapter should be on the Tx line of the analyzer and reverse for the Tx line from the adapter.

I hope this information helps!