RX3i Modbus via ETM

jimwawar47

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Join Date
Dec 2010
Location
Halifax
Posts
21
How can a timeout be handled in an RX3i communicating Modbus TCP over an ETM?

I have a CRU320 (firmware 8.95) setup in dual redundant mode communicating via an ETM001 (firmware 6.43, J module so can't upgrade to 7+) over Modbus TCP to a Moxa 3170 gateway. The Moxa 3170 gateway communicates to an iGARD controller (DSP-OHMNI) over 4 wire RS485.

Using Moxa MGate Manager (v1.17) software I am able to monitor the communications on the gateway. I can see the incoming TCP request from the CRU320, followed by an outgoing 485 request to iGARD. Most of the time I can see a return 485 from the iGARD and a return TCP to the CRU320. Sometimes there is no return from the iGARD over 485 and no return from the Moxa over TCP. My read status remains a 0. Using ModScan I can see the missed the returns on the MGate Manager. ModScan times out and discards the message with no reply then sends again.

I haven't been able to find a Timeout setting on the RX3i or a way to discard sent messages. In GFK-2224 9.5.4 I thought I could send a close command to flush the system and reopen the channel. When I try to send the close command the status changes to the error AB90 (A COMMREQ was discarded because the application program issued the COMMREQ before the COMMREQ Status Word for the previous COMMREQ was set.)

The customer isn't willing to pay for an upgrade to CPE330.

Thanks in advance for any help,
Jim
 
In case someone stumbles on this down the road. I wasn't able to find how to flush the RX3i Modbus queue. As a temporary fix I did a timer to wait for the return from Moxa. If the timer expired I dropped the connection to flush the Modbus queue and then reconnected.

The permanent fix was found by reading the Moxa manual. The Moxa gateway has TCP timeout setting. If the RTU side hasn't responded it sends either a timeout or device not online message. I received both. Error handling in the ladder handles the rest.
 

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