Adding a Master-Wake-Up signalto the rs-485 bus ?

[henninggs]

Adding a Master-Wake-Up signal to the rs-485 bus ?

Hi.

I have a rs-485 network with one master and multiple slaves. A slave is only speaking if spoken to by the master.

Let's say I have a slave that monitors a sensor. If the sensor value gets outside a predefined range the master must be notified within a few seconds. Chances are that the sensor will get outside the range just once or twice a year.

One way to implement this, is to have the master polling the slave every few seconds asking if the sensor is outside the range. This would mean a lot of unnecessary traffic since 99.999% of the time the answer would be "no".

So I was thinking: Since the network is wired with cat5e cable I have some wires not in use by the rs485 bus. If I could use one of these wires to signal the master that it's time to poll the slaves for a status update I could greatly reduce traffic on the bus. (An properly also the overall power consumption)

The signal could just be a HIGH on the spare wire and it would not be a problem if two slaves sets it high at the same time: As soon as the master gets the signal it goes into polling all slaves asking for their status.

What do you think of this idea and how could it be implemented on the hardware side ?
Are there any simpler or better solutions ?
Any thoughts is greatly appreciated.

 

[FvM]

I would rather think of allowing a small piece of multi-master operation by allowing unsolicited messages from the slaves. As in any multi-master systems, possible collisions have to be handled.

I don't clearly see the point why you don't want to poll the devices, by the ways. Probably it's still the most economic way. If you want to reduce the communication amount, the master can signal a time window for optional answers, that is only utilized in case of pending messages.

 

[henninggs]

...allowing unsolicited messages from the slaves. As in any multi-master systems, possible collisions have to be handled.

It'll be a single-master system.

. I don't clearly see the point why you don't want to poll the devices.

Well, to save bandwidth and power. In a system where 99% of the traffic and a large part of the power consumption is "wasted" it just seamed a good idea.
(Haven't done the math on the power consumption part, just guessing)

If you want to reduce the communication amount, the master can signal a time window for optional answers, that is only utilized in case of pending messages.

That is a very good idea!

 

[henninggs]

Would this work for signaling the master ?

 

[henninggs]

Will it work ?