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MODBUS Comm Link Issue with Multiple Slaves

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MODBUS Comm Link Issue with Multiple Slaves

This question was originally posted on DCIM Support by Yang Gao on 2016-03-31


We have 4 slave generators which communicate with DCE via an internet gateway device (ComAp's IB-NT). The comm. link for the slave devices went up and down. It appears that only one slave is up at a time. What could be the problem and what is the solution. Can DCE works with multiple slaves via gateway device?

 

I tried to change the timeout to different values and see no difference.

(CID:105470162)

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Re: MODBUS Comm Link Issue with Multiple Slaves

This answer was originally posted on DCIM Support by Steven Marchetti on 2016-03-31


Hi Yang,

 

DCE is definitely capable of communicating with multiple slave devices on a single gateway. The only gateway we have tested and support at this point is the modbus wired to modbus over IP gateway TSXETG100 but similar devices should work.  Have you tried polling the different devices manually from the same subnet as DCE?

 

You may also want to look at the baud rates used on the wired side of the gateway. I believe higher baud rate on the devices and gateway may allow for a faster transfer of data and potentially less comm issues. You can test faster / slower rates.

 

If you have any other systems communicating to these devices, that could also cause comms issues. If you have such a configuration, stop the other system (serial or over IP) from monitoring the devices for at least a test period. 

 

You may also be able to run a packet capture to see if the devices are responding or if DCE is even polling. SOme gateways also have troubleshooting utilities that can log this type of data but you'd need to check their manual or support options for that info.

 

Steve

(CID:105470172)

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Re: MODBUS Comm Link Issue with Multiple Slaves

This comment was originally posted on DCIM Support by Yang Gao on 2016-03-31


DCE has no problem to talk to a single MODBUS device (without gateway). DCE may have problem to talk to multiple slave device with common gateway. Here is my understanding about the problem. When DCE send a polling request to a device, it requires to establish TCP/IP communication with the remote device. Once a reply is received, it clear the TCP/IP communication with remote device. During TCP/IP session, another polling request from different server to the same remote device will be rejected. In the case of multiple slave devices on the same gateway, if DCE sends polling requests for all slave devices in a relative close time interval, some requests will be rejected if the previous TCP/IP session has not been cleared. The problem is user can not control the polling interval among the devices. I tried with 2 slave devices with polling interval between the 2 devices about 30 to 60 seconds and the global scanning at 2 minute, I had no comm. link issue. is there any way DCE can set or control the polling interval among the slave devices with common gateway.?

(CID:105470194)

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Re: MODBUS Comm Link Issue with Multiple Slaves

This comment was originally posted on DCIM Support by Steven Marchetti on 2016-03-31


Hi Yang, That may be the case and again that's why I suggested looking at the gateway. There is no way to set polling delays between different devices. If the gateway you're using is having issues when polling another device within a few seconds of the first, then perhaps another type of gateway or multiple gateways are required. Again, issues like that can in some cases be effected by changing baud rates. Since DCE is meant to be capable of monitoring up to 4025 devices, making it possible to create delays of 20 or 30 seconds between polls would be impractical and would itself cause issues. There would also be quite a bit more code required to make such delays possible between different devices on the same IP/gateway. Steve

(CID:105470196)

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