How to create an Modbus-TCP alarm status dashboard

Industrial
12

Funny thing - using a modbus slave emulator, if i request over 127 items, i start getting strange length responses in the debug output from node-red-contrib-modbus

The slave application seems to send all the data that i request. For example, I requested 150 items & the slave responded with the right amount of data...

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but the modbus flex getter only shows me the first 22 items...
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Another oddity is Wireshark doesn't correctly recognise the response...

Normal wireshark response (127 items)...

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When i request 150 items...

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And this online modbus packet parser doesnt like the response.

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... which leads me to think ...

  • The response is actually NG or out of spec?
  • The wireshark filter and the web site checker tool is old / limited / broken?

I dont have the energy to scour the modbus spec so if anyone familiar with the modbus tcp specification wants to analyse the following request and response packets please do...

node-red --> slave

000100000006010300000096

slave --> node-red

00010000012f01032c00010002000300000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000005f0060000000000000000000000000000000000000000000000000000000000000007c007d007e007f00800081000000000000000000000000000000000000007f0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000


Possible work arounds...

  1. Do 4 sequential requests for 0~99 then 100~199 then 200~299 then 300~349 (you could do it in 3 reads but aligning to the 0, 100, 200 and 300 mark will make parsing simpler)
  2. DIY it - send 00010000000601030000015e over TCP to your slave & parse the result yourself

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Demo DIY modbus TCP flow...
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Results...

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flow...

[{"id":"81661067.26948","type":"tcp request","z":"5e6c8b.7f38b374","server":"192.168.1.59","port":"502","out":"sit","splitc":" ","name":"","x":1908,"y":208,"wires":[["edb7a472.6eebc8"]]},{"id":"f77e01bf.42f16","type":"inject","z":"5e6c8b.7f38b374","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"00010000000601030000015e","payloadType":"str","x":1872,"y":144,"wires":[["50df09a0.9d44e8"]]},{"id":"50df09a0.9d44e8","type":"buffer-maker","z":"5e6c8b.7f38b374","name":"","specification":"spec","specificationType":"ui","items":[{"name":"item1","type":"hex","length":-1,"dataType":"msg","data":"payload"}],"swap1":"","swap2":"","swap3":"","swap1Type":"swap","swap2Type":"swap","swap3Type":"swap","msgProperty":"payload","msgPropertyType":"str","x":2054,"y":144,"wires":[["cef58d63.4cb6","81661067.26948"]]},{"id":"f662fe41.9f3f3","type":"debug","z":"5e6c8b.7f38b374","name":"modbus response","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":2282,"y":208,"wires":[]},{"id":"cef58d63.4cb6","type":"debug","z":"5e6c8b.7f38b374","name":"modbus request","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":2272,"y":144,"wires":[]},{"id":"edb7a472.6eebc8","type":"buffer-parser","z":"5e6c8b.7f38b374","name":"","data":"payload","dataType":"msg","specification":"spec","specificationType":"ui","items":[{"type":"uint16be","name":"data","offset":9,"length":-1,"offsetbit":0,"scale":"1","mask":""},{"type":"buffer","name":"buffer","offset":9,"length":-1,"offsetbit":0,"scale":"1","mask":""}],"swap1":"","swap2":"","swap3":"","swap1Type":"swap","swap2Type":"swap","swap3Type":"swap","msgProperty":"payload","msgPropertyType":"str","resultType":"keyvalue","resultTypeType":"output","multipleResult":false,"fanOutMultipleResult":false,"setTopic":true,"outputs":1,"x":2102,"y":208,"wires":[["f662fe41.9f3f3"]]}]

NOTE: Here be dragons. The above flow does not check headers, does not do any error checking & i have no idea how it will handle disconnections. Feel free to use it or not but i would recommend at least checking the modbus headers and response data length at minimum before accepting the data as good