Modbus TCP-node - (16 bits register neg number & 32 bit register reading)

[Luis GCU]

Hello 

First of all  I want to say thanks to all the team and developers behind node-red, its simply a great idea and perfect tool for people like me that are not so advanced in high level programmings platform.

I am doing  several test with node-red so far  work great,  I just need more time reading and learning, very soon I will be posting some of those test project.

Just now I am testing  the   modbus TCP  Node communication  with AC-Servodrives , and the communication work fine , the only issue that I  have now is reading 16bits holding register when the value is negative the reading display is 65535, I also notice that I can't read 32 bits register using the offset, not sure is there is any workaround or just that  the modbus TCP node is not fully implemented at this point.

regards,

[Julian Knight]

This is where it helps to be old(er)! Since handling numbers at the bit level used to be common. You are treating the number as an unsigned integer rather than a signed one. The difference being that the most significant BIT in a signed number say whether the number is positive or negative. So you really have 15bits for the number and one for the sign.

[Luis GCU]

you right mate, the question is how should  I proceed to read a sint −32,768 to 32,767 - on node red modbus tcp node .. I don't see any  option to select sint  or int word. 

I use other HMI platform where you have those option .. but that node seems to do not have tha yet.. 

Thanks 

[Julian Knight]

Well, if the output is a buffer then node.js has some buffer read tools that would probably do the job for you: https://nodejs.org/api/buffer.html#buffer_buf_readint16be_offset_noassert

So maybe you could pass the buffer to a function node and then use the readint function to make sense of it?

[Dave C-J]

Or talk nicely to the developer of the node... It does get updated fairly regularly.

[Luis GCU]

Thanks you guys for your advice,  I will try to follow Julian suggestions. 

[Paul]

Did you ever sort this out, I am having the same problem with a solar inverter

Paul

[Luis GCU]

Hello,

there is new modbus TCP  node  ( maybe not new but I notice  that one very recently) with that node  you can do a lot  manipulation with the buffer, I installed and tested sometime ago and it worked fine for me.. 

make sure you got this node .. https://flows.nodered.org/node/node-red-contrib-modbus

Hope this helps.

[Leif Neland]

I'm using the serial modbus, so I don't know if this is appliccable.

I read a 32 bit number, and then r=msg.payload.readUInt32BE(0)

So your 16 bit should be converted with msg.payload.readInt16BE (or readInt16LE)

[{"id":"b9fbecfb.dace4","type":"modbusSerial in","z":"cda43e91.58161","port":"54dc1f20.e61e3","slaves":"10","start":"286","count":"2","dtype":"holding","period":"5","topic":"Modbus","name":"B","x":105,"y":220,"wires":[["18bec1f8.7cdd5e"]]},{"id":"18bec1f8.7cdd5e","type":"function","z":"cda43e91.58161","name":"","func":"r=msg.payload.readUInt32BE(0)\nmsg.payload=r/100;\nreturn msg;","outputs":1,"noerr":0,"x":247,"y":217,"wires":[["a88dd1ad.36e6d"]]},{"id":"a88dd1ad.36e6d","type":"mqtt out","z":"cda43e91.58161","name":"","topic":"VarmeKWH","qos":"2","retain":"true","broker":"605c9cbd.0864b4","x":435.7869110107422,"y":223.58521270751953,"wires":[]},{"id":"54dc1f20.e61e3","type":"modbusSerialConfig","z":"","port":"/dev/modbus","baud":"2400","data":"8","parity":"even","stop":"1","name":""},{"id":"605c9cbd.0864b4","type":"mqtt-broker","z":"","broker":"192.168.1.254","port":"1883","clientid":"","usetls":false,"compatmode":true,"keepalive":"60","cleansession":true,"willTopic":"","willQos":"0","willPayload":"","birthTopic":"","birthQos":"0","birthPayload":""}]