# Modbus TCP failing on more than 1 slave device read **URL:** \ **Category:** Ignition\ **Tags:** modbus, perspective\ **Created:** [September 18, 2026, 11:44am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129 "2026-09-18T11:44:49Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 18, 2026, 11:44am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/1 "2026-09-18T11:44:49Z")
I have a USR DR302 rs485 to tcp gateway(converter). I connect it to ignition using the Modbus TCP driver but noticed that when I try to request tag data for more than two slaves it starts giving me bad tag value and timeout error. If I poll one slave at a time it works perfectly. Has anyone come across this issue before ? ---
**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\ **Post date:** [September 18, 2026, 12:54pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/2 "2026-09-18T12:54:37Z")
Make sure concurrency is set to one for such devices. ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 18, 2026, 1:05pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/3 "2026-09-18T13:05:57Z")
Yes , It is set to one. And same issues persists ---
**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\ **Post date:** [September 18, 2026, 1:42pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/4 "2026-09-18T13:42:50Z")
Just looked up the USR DR302. From the Amazon description, it isn't a modbus gateway. It is simply a serial rs485 to ethernet gateway. That means it needs the Modbus RTU over TCP driver, not the Modbus TCP driver. IA's modbus RTU driver doesn't even try to enforce any RTU timing specs. You may need either a true MB Gateway device, or possibly my alternate driver. ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 18, 2026, 1:55pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/5 "2026-09-18T13:55:54Z")
It does have option where you tick Modbus RTU to TCP. I ticked that as well so it could convert properly. Only issue is when I try to poll more than 1 slave, it throws the readinput register timeout errors. ---
**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\ **Post date:** [September 18, 2026, 1:59pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/6 "2026-09-18T13:59:45Z")
> [@william.agbo](#): > > It does have option Perhaps that option simply doesn't work. There's a review on Amazon from someone unsuccessfully trying Modbus TCP and getting no help from USR. I would presume USR is confused by the difference between Modbus RTU Over TCP and Modbus TCP. Or simple incompetence. ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 18, 2026, 2:15pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/7 "2026-09-18T14:15:33Z")
So what can I do in this case? The client already installed them ---
**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\ **Post date:** [September 18, 2026, 2:47pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/8 "2026-09-18T14:47:09Z")
Reconfigure to the Modbus RTU over TCP driver, and turn off the device's Modbus TCP function. Meanwhile, you should also verify that your devices on the RS485 bus are compatible with each other. Enable tags for the different units one at a time, to ensure each one functions correctly when it is the only one responding. Then try the various combinations of units together, to see if there's any clashes. A wireshark capture in concert with a serial line snooper may also help. ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 18, 2026, 6:41pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/9 "2026-09-18T18:41:01Z")
Thank you. I will try these ---
**Author:** ![David\_Stone](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/david_stone/32/79749_2.png) [@David\_Stone](https://forum.inductiveautomation.com/u/David_Stone)\ **Post date:** [September 19, 2026, 6:33am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/10 "2026-09-19T06:33:12Z")
I have used the USR units before, they do work when converting RTU to TCP but they definitely aren’t reliable. I ended up relegating them to low rate data from power meters only. ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 19, 2026, 6:47am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/11 "2026-09-19T06:47:08Z")
Did you have issues when polling more than one slave at a time? It shows readinput timeout (overload) when I add a second or 3rd slave. Just about 10tags from each Power meter ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 19, 2026, 6:48am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/12 "2026-09-19T06:48:01Z")
Also, which driver did you use? Modbus TCP or Modbus RTU over tcp? ---
**Author:** ![David\_Stone](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/david_stone/32/79749_2.png) [@David\_Stone](https://forum.inductiveautomation.com/u/David_Stone)\ **Post date:** [September 19, 2026, 7:41am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/13 "2026-09-19T07:41:29Z")
I had many issues. It’s sensitive to termination, interference and speed. What speed are you running and how many registers per device? I used Modbus TCP every time. ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 20, 2026, 9:58pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/14 "2026-09-20T21:58:11Z")
I am using 19200 and polling about 10 float variables(20 registers) per ID. i also have about 10 devices to 22 devices connected to one USR gateway ---
**Author:** ![David\_Stone](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/david_stone/32/79749_2.png) [@David\_Stone](https://forum.inductiveautomation.com/u/David_Stone)\ **Post date:** [September 20, 2026, 10:12pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/15 "2026-09-20T22:12:52Z")
What update rate are the tags set to in Ignition? ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 20, 2026, 10:45pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/16 "2026-09-20T22:45:53Z")
1s ---
**Author:** ![David\_Stone](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/david_stone/32/79749_2.png) [@David\_Stone](https://forum.inductiveautomation.com/u/David_Stone)\ **Post date:** [September 21, 2026, 8:59am UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/17 "2026-09-21T08:59:48Z")
Try setting it to 10s ---
**Author:** ![william.agbo](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@william.agbo](https://forum.inductiveautomation.com/u/william.agbo)\ **Post date:** [September 22, 2026, 9:32pm UTC](https://forum.inductiveautomation.com/t/modbus-tcp-failing-on-more-than-1-slave-device-read/117129/18 "2026-09-22T21:32:28Z")
Thanks everyone for the reply. What worked for me was polling each PQM ID one at a time for each gateway. so i created different driven tag groups for that.