# Modbus rtu over tcp read failure

**URL:** <https://forum.inductiveautomation.com/t/modbus-rtu-over-tcp-read-failure/35815>\
**Category:** Ignition\
**Tags:** ignition80\
**Created:** [June 19, 2020, 5:34am UTC](https://forum.inductiveautomation.com/t/modbus-rtu-over-tcp-read-failure/35815 "2020-06-19T05:34:24Z")\
**Posts on this page:** 1\
**Showing post:** 13

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**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:** [June 21, 2020, 5:51pm UTC](https://forum.inductiveautomation.com/t/modbus-rtu-over-tcp-read-failure/35815/13 "2020-06-21T17:51:39Z")

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Your network load is not the problem. Based on your JSON export, most of your tags are contiguous, words 22-35, with three others: 6-7, 44-45, and 66-67. Ideally, you need four requests to read these blocks, satisfying the ten float tags. Per the modbus specification (PI-MBUS-300 from [modbus.org](http://modbus.org)) these requests and their responses should take 62.5ms at 19200-N-8-1 (including the inter-message silent time on the wire). Twenty devices is a modest overload–you won’t get one-second timing.

I believe Ignition will make the requests in the pattern above if the “Span Gaps” setting is turned off in the driver. I suspect that your devices have registers (other than your ten) that respond poorly, and spanning gaps will read unexpected registers. Spanning from word 6 to word 67 would take 75ms, but would read up to 31 floats, which is why it is default “on”. Try turning that setting off.

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