Hi folks.

Twelve wellpad RTUs polled by Modbus TCP over LTE modems, one modem per site on a private APN. Single master in the SCADA server, 5 s poll cycle, 2 s timeout per site.

Half the sites time out every cycle, and it's a different half each time. Ping from the server to any modem is 300 to 900 ms, sometimes 1.5 s. The RTUs themselves are fine, a laptop on the pad reads them instantly.

Somebody told me the far pads were marginal on signal, so we swapped the antennas at the two worst for directional ones and RSSI came up ten dB, but the timeouts didn't change at all. Carrier says their network is fine and so are the modems. Is this the modems, the carrier, or my polling? Is a 2 s timeout just never going to work on a link that pings at 900 ms, or am I at the wrong end of this?