# Modbus client not translating tag into correct floating-point format **URL:** \ **Category:** Ignition\ **Created:** [March 10, 2022, 9:34pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087 "2022-03-10T21:34:39Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 9:34pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/1 "2022-03-10T21:34:39Z")
I have a modus server spun up using pymodbus, it pulls data from a CSV and updates the holding registers once every second (all values are 32-bit float). I’ve written a basic client for this server, and verified that values pulled from the csv, pushed to the server and then pulled “on the other side” by the client match what was originally in the CSV. However, when I connect to this server using the Modus TCP driver in Ignition Edge, it can’t seem to decode the value even when I explicitly put it in as a float - it seems to be instead providing the numerical value of the first 16-bit word, or some weird intermediate values. This is what my tag (right now I’m just trying to get one) looks like in designer: ![Screen Shot 2022-03-10 at 4.31.04 PM](https://us1.discourse-cdn.com/inductiveautomation/original/3X/e/a/eaa05d612c7b67c538997833212071126d34181d.png) This is what the overall designer window is showing me: ![Screen Shot 2022-03-10 at 4.32.03 PM](https://us1.discourse-cdn.com/inductiveautomation/original/3X/1/6/162af5a023c354f0a5c541ad6eddf7137a870c58.png) And this is what I see on the gateway side: ![Screen Shot 2022-03-10 at 4.34.02 PM](https://us1.discourse-cdn.com/inductiveautomation/original/3X/2/1/21fd26e63bd04f3d2b850f40533340a022d97e85.png) ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 9:35pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/2 "2022-03-10T21:35:19Z")
You’re likely off by one register or need to enable the “Swap Words” setting, or both. ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 9:40pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/3 "2022-03-10T21:40:09Z")
It’s unlikely to be an off-by-one error, I checked and compared the modbus requests received when using my hand-made client and when using Ignition, they are exactly the same. As far as the “Swap Words” setting, could you tell me where that is? I don’t see that in either the gateway or the Designer. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 9:40pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/4 "2022-03-10T21:40:56Z")
It’s in the Gateway under the advanced settings for the Modbus device you created. ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 9:43pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/5 "2022-03-10T21:43:38Z")
Is it “Reverse Word Order”? Because I just toggled that and it doesn’t seem to have had any effect. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 9:44pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/6 "2022-03-10T21:44:46Z")
Yeah, that’s it. If you can get a Wireshark capture or turn the “ReadHoldingRegistersRequest” logger to TRACE level you’ll be able to see exactly what value your server is returning in response to exactly what request. [https://docs.inductiveautomation.com/display/DOC81/Diagnostics+-+Logs#DiagnosticsLogs-ChangingLoggingLevels](https://docs.inductiveautomation.com/display/DOC81/Diagnostics+-+Logs#DiagnosticsLogs-ChangingLoggingLevels) You’ve specified `HRF1` as your address, so you should expect to see a read holding registers request for 2 registers at protocol address 0. (Ignition is 1-based by default, like the Modbus spec describes, however at the protocol level Modbus is 0-based…). You should see 4 bytes for those 2 registers in return. The default expectation before you toggled that setting was that they will combine form a 4-byte float in big-endian byte order. Reverse Word Order just does exactly what it sounds like, swaps the registers before looking at the bytes. [HR1][HR2] → [HR2][HR1]. ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 9:59pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/7 "2022-03-10T21:59:28Z")
Right, I suspected as much. On the modbus server that I spun up, here’s what I see in the logs: ```auto 2022-03-10 16:52:01,699 MainThread DEBUG asynchronous :64 Data Received: 0x2 0x47 0x0 0x0 0x0 0x6 0x0 0x3 0x0 0x0 0x0 0x2 2022-03-10 16:52:01,699 MainThread DEBUG socket_framer :147 Processing: 0x2 0x47 0x0 0x0 0x0 0x6 0x0 0x3 0x0 0x0 0x0 0x2 2022-03-10 16:52:01,699 MainThread DEBUG factory :137 Factory Request[ReadHoldingRegistersRequest: 3] 2022-03-10 16:52:01,699 MainThread DEBUG context :63 validate: fc-[3] address-1: count-2 2022-03-10 16:52:01,699 MainThread DEBUG context :77 getValues fc-[3] address-1: count-2 2022-03-10 16:52:01,700 MainThread DEBUG asynchronous :102 send: b'024700000007000304733ebbd5' ``` It’s not from the IA internals, but hopefully should be understandable - the modbus request we see on the first line looks exactly like I’d expect, starting at 0x0 0x0 and ending at 0x0 0x2. The logs from my handmade client show the exact same request, minus the synchronization bytes at the beginning. I think I figured out how to turn on the Ignition logger for ReadHoldingRegistersRequest (or at least filter out for specifically that logger) but it doesn’t seem to be showing every request explicitly, just the ones that failed. Even with me toggling the logging level to TRACE. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 10:00pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/8 "2022-03-10T22:00:47Z")
You need to use the gear icon in the upper right to find the logger and set its level, not the filter you get presented with on the main log view. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 10:05pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/9 "2022-03-10T22:05:50Z")
What value are you expecting? Even from your logs I can see the server is returning `733ebbd5`: ```python >>> import struct >>> struct.unpack('!f', '733ebbd5'.decode('hex'))[0] 1.5111482075746394e+31 ``` ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 10:06pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/10 "2022-03-10T22:06:37Z")
Gotcha. Here’s a screenshot of the logs: ![Screen Shot 2022-03-10 at 5.03.28 PM](https://us1.discourse-cdn.com/inductiveautomation/original/3X/a/f/afb33cfe328428f29be2cf0798024f0b7ac0fec1.png) ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 10:08pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/11 "2022-03-10T22:08:09Z")
Something in the range of 0-1 - not entirely sure where the issue is happening, because it looks to be receiving the exact same request but working fine in my handmade client… But I’m explicitly doing the unpacking/decoding on that one. I’ll have to do some deeper investigation, give me a little bit… ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 10:08pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/12 "2022-03-10T22:08:23Z")
Okay so the value is changing each response, but it still looks like your server isn’t returning anything sensible. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 10:09pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/13 "2022-03-10T22:09:13Z")
Ignition uses unit/slave id 0 by default - is your client also using 0 or using 1 or something else? ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 10:19pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/14 "2022-03-10T22:19:10Z")
It might be using unit 1, actually… pymodbus isn’t that clear on the defaults. I just tried to check, and I actually got the correct expected responses using both unit\_id=0 and unit\_id=1 (this was in my handmade client). That’s a little unexpected… ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [March 10, 2022, 10:22pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/15 "2022-03-10T22:22:54Z")
If the requests and responses are the same then I can only imagine your client interprets the bytes in a different order. Given `733ebbd5` the only order that yields a “normal” looking value in [0,1] is BADC, where Ignition expects ABCD (big endian) or CDAB (word swapped). ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 10, 2022, 10:33pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/16 "2022-03-10T22:33:17Z")
I’ll follow up on this later - you’ve given me some good information and I have a hunch you may be right about the ordering issue. Currently investigating. ---
**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:** [March 11, 2022, 1:20am UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/17 "2022-03-11T01:20:25Z")
Tip: Modbus _ **defines** _ the order on the wire for 16-bit quantities to be big-endian. Send some 16-bit integers and see how they come out. ---
**Author:** ![fshriver](https://avatars.discourse-cdn.com/v4/letter/f/db5fbb/32.png) [@fshriver](https://forum.inductiveautomation.com/u/fshriver)\ **Post date:** [March 11, 2022, 11:38pm UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/18 "2022-03-11T23:38:47Z")
Finishing this issue for the sake of posterity (hopefully somebody will find this useful in the future): Apparently pymodbus by default saves word order as Big Endian and byte order as Little Endian, exactly the opposite of what works for Ignition either default or word-swapped. Unsure if there’s a way to account for this in Ignition, but hopefully this knowledge is useful for someone else in the future. ---
**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:** [March 12, 2022, 12:01am UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/19 "2022-03-12T00:01:38Z")
> [@fshriver](#): > > to account for this in Ignition Nope. pymodbus is just wrong. The modbus spec leaves a great deal unstated for non-16-bit quantities, but those are always big-endian in the payload. ---
**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\ **Post date:** [March 12, 2022, 12:53am UTC](https://forum.inductiveautomation.com/t/modbus-client-not-translating-tag-into-correct-floating-point-format/57087/20 "2022-03-12T00:53:59Z")
It looks like you can force PyModbus to use different packing, somehow: >