Remote pump station over a 4G router: values freeze and the channel drops

[SystemPulse]

We added a remote pump station to Omnivex SCADA. The PLC there is reachable through an industrial 4G router with a VPN, and we read it with a Modbus TCP channel. In the office, ping times to the PLC vary between 80 ms and sometimes more than 1 second.
The local stations work perfectly, but the remote station's values freeze for a while several times an hour, and sometimes the channel seems to drop completely before it recovers. The channel was created with the same settings as our local channels. Which settings should I change for a slow, unstable connection like this?

[Technical Support]

For mobile links the most important setting is the channel's Request Time. On a Modbus TCP channel, Request Time is the time in milliseconds that SCADA waits for a response from the device. A value that works on a local network (where answers come in a few milliseconds) is too short for 4G: every slow answer is treated as no answer, and the values stay at their last state until the next successful request.
Recommended steps
1. Open the Channel Info of the remote channel and increase Request Time clearly above the worst response time you measure, for example 2000–3000 ms for a 4G link. Click Add/save and restart communication.
2. Use few, larger frames for the remote PLC. Every frame is a separate request over the slow link. Group consecutive registers into one frame (Start Address + Quantity) instead of many small frames.
3. Read only what you need from the remote site. Tags that are only used for commissioning can be set inactive with the Active field.
4. Keep the remote station on its own channel, so a slow answer there doesn't delay your local devices.
On the network side
- Check the router's signal quality and whether the VPN tunnel restarts at fixed intervals. Regular freezes at the same minute often point to a tunnel or provider reconnect.
- Make sure no other system polls the same PLC through the router at the same time.
If the values still freeze after these changes, send us the channel settings and the times of the freezes, and we will check them with you.
Documentation: https://greensoft.ai/documentation/o...ion-protocols/