Serial port in multithreaded flow - how would you solve it?

Hardware
1

Hi there

I'm currently designing a flow where a serial port is used and need some hint:

Serial port is natively a single-threaded thing - e.g. only one character can be sent a a time and the same is for reception. In my flow I communicate with a simple device, which responds to requests. E.g. I need to send a message like "give me value at address X" to the serial port and device responses "The value at address X is Y". I can also send a command "Write Value Z to Address X". Response would be "Written Value Z to address X".
My problem is that such requests in my flow happening asynchronously and nondeterministically from different sources - dashboards, when user presses some button, or periodically e.g when I need to poll some value. So I need a way to serialize the requests to send them one by one into Port and also, ideally, limit the speed not by time (which I think is obviously possible using delay node), but by waiting for a response from device, before next request is sent. So a kind of queue.

Does anyone have solved such task already? What would be the typical solution here, ideally using only standard nodes without functions?
Thanks.

2

There is already a core node that may do much of this for you.

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image626Ă—505 19.7 KB

You may need to tune the interval to your needs.

Otherwise, one of the contributed "gate" nodes would let you block until getting a result (you could, of course, build a flow yourself as an alternative). Not sure if any of them have a queue feature though.

3

Thanks, just updated my message about delay node - this is known to me. The question was mostly how to wait for a response from serial port before next message is allowed to transmit.

4

The idea would be to keep a flag context variable and a queue context variable.

In your flow, when a msg is sent, change the flag. In the flow that sends the msg, check the flag before sending. If the flag is set to block then add the msg to the queue instead of sending.

You would also need something to watch for the flag changing though. You might be able to use the status node to help with that. But I would normally just use MQTT since you can subscribe to changes. Sadly, the context variable library in Node-RED does not have a subscribe capability - I've always meant to get round to writing one but never seem to quite get the time!

Check the flows site though to see if any contributed nodes do what you want.

Sometihng like this?

5

This flow shows how you can use the core Delay node Flush mechanism to queue messages and release them as each message action is completed.

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Open the Comment node to get instructions on how to use it.
The Copy button below the flow will copy it to the paste buffer.

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6

A semaphore is useful for these scenarios.

Here is one implemented by one of our regulars @marcus-j-davies:

It has built-in example flows too (use CTRL-I to select an example)

3 Likes
7

Alternatively, I believe that the Serial Request node will handle that for you with no additional logic.

1 Like