Send and response with serial

Good day!
I want to do communication node-red (raspbery 3+) with arudino using rs485 protocol and have some questions.
I can send and receive messages through the serial port node. It is very easy with node-red.
But I want to slightly modify my "protocol" and want to make response of receipt and sending of data. But I dont understand how.
For example, I send data to several arduinos and set the send flag to 1 (Flow context) using the Set node. Then I receive a response from the arduino like "1OK", "2OK"... etc., where the first char is the ID of the arduino, "OK" - response of data receipt. I need:

  1. Check if 2 and 3 char to equal "OK";
  2. If it equal, then check what the first character is;
  3. Set the corresponding ID flag equal to 1.

How to do it? I think about function, but it little hard for me.
Can I do this without using functions? With standard nodes?

If you have control over the format of data then i would recommend sending JSON formatted data (then its very simple to convert the strings into usable JS objects).

Also, I would decide on a terminator constant (like 0x3 ETX) so that you can safely and reliably split serial strings.

As for confirmation, many protocols employ a standard sequence like

Send SOH SEQ STX DATA ETX EOT
Reponse SOH SEQ ACK EOT

or perhaps...

Send STX SEQ ENQ DATA ETX
Reponse STX SEQ ACK ETX

it is up to you how you determine your protocol - you could even look up and utelise a basic ASCII Protocol - like modbus

where...


SOH  = Start of Header
SEQ  = a message ID / Sequence number (so that you can confirm responses against that number)
STX  = Start of Text/data
DATA = standard JSON string with your DATA in
ETX  = End of Text/data
EOT  = End of transmission
ACK  = GOOD Acknoledgement

See the ASCII table for value of SOH, STX, ETX, ACK etc

In otherwords, if you send SEQ 77 with some DATA, you MUST get a reply of 77 ACK

If you receive 78 - you are out of sequnece,
If you receive 77 NAK - the data was recieved but the other device "didnt like it" so replied with a "negative ACK"


If you are not familiar with JSON - dont worry, it is quite easy to produce and very useful in this (javascript) environment as it is a native data transfer format. (just ask if you need further direction)

I will try to explain with a picture what I want to do.

Ok, step-by-step.
I reciver buffer data from serial. For example [2,1,3]. I want to check that the third byte is equal to the sum of the first and second bytes. How can I do it?

Assuming it is in the payload then
if (msg.payload[3] ==msg.payload[1]+msg.payload[2])

It is code for function node?
Can i do it with standart node? Switch, Set etc...

You can do it in a switch or change node using JSONata
i.e.

[{"id":"e80c63e9.b052e8","type":"inject","z":"8d22ae29.7df6d","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[2,1,3]","payloadType":"bin","x":130,"y":3860,"wires":[["6b997dfc.dbcb6c","2756679c.c38ed8"]]},{"id":"6b997dfc.dbcb6c","type":"switch","z":"8d22ae29.7df6d","name":"","property":"payload.data[2]","propertyType":"msg","rules":[{"t":"eq","v":"payload.data[0] + payload.data[1]","vt":"jsonata"}],"checkall":"true","repair":false,"outputs":1,"x":290,"y":3920,"wires":[["2756679c.c38ed8"]]},{"id":"2756679c.c38ed8","type":"debug","z":"8d22ae29.7df6d","name":"","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload","targetType":"msg","statusVal":"$keys(payload)","statusType":"auto","x":500,"y":3920,"wires":[]}]