can not intiate connection with rtu connected to gprs modem


Thread Starter

ahmed salah

i have rtu connected to gprs modem with fixed ip. i am trying to poll rtu using mdbus but i got connection failed. but when i try to ping rtu gprs modem ip address from the pc which i pool from i can reach its ip. so what is the problem???

Lynn A Linse

Your problem is likely the time delays. Cellular IP systems routinely require a round trip delay of 3-4 seconds ... of course at times it also works round-trip in 0.4 seconds (400 milliseconds) ... and a few times a day it could take 10 to 15 seconds. I've had customers see 38 to 49 second delays on "bad days" - voice calls ALWAYS have priority over data with 2G and 2.5G systems. So if you have a "normal" Modbus/RTU slave timeout of 1 second, I highly doubt you'll ever get it to work. Start with a 15 second timeout. Unfortunately, even if your application waits 15 seconds, the Windows default "socket open" timeout is I think only 5
seconds so it may be possible you cannot even open a socket in such a short time.

Remember, the GPRS modem is "always up" and "always connected" - however the cell tower will deallocate the active data transfer resources if our GPRS device is idle for between 3 to 40 seconds (depends on "standard" being used). This is part of the cause of the initial
delay. Take a look at this PING trace from one of my GPRS/EDGE devices - this is actually a very GOOD response but notice how the first 2 take longer than the last. This pattern is "normal" - the more active your GSM node, the more steady is the resource allocated between tower and cell device.

Reply from bytes=32 time=1214ms TTL=240
Reply from bytes=32 time=670ms TTL=240
Reply from bytes=32 time=481ms TTL=240
Reply from bytes=32 time=420ms TTL=240

Best regards
- Lynn A Linse,