[sdiy] dsPIC UART for multi-processor comms

[Jay Schwichtenberg]

I would expect like Richie said multiple TX lines tied together. Or maybe some other low logic line is tied to the TX.

Another thing might be the polarity programming. Don't know about the dsPICs but all the other chips I've worked with have the ability to program the TX/RX polarity to support inverting and non-inverting drivers.

Try each of your satellite processors individually and see if one or all cause the problem. If all do then I'd expect a software problems.

If you have a scope put that on it, that should tell you if you have an electrical problem. A logic analyzer would tell you if you had a data problem so that isn't good here.

Jay S.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom Wiltshire
Sent: Sunday, April 22, 2018 8:27 AM
To: synth-diy at synth-diy org
Subject: [sdiy] dsPIC UART for multi-processor comms

Hi all,

I’m currently experimenting with communications between multiple dsPICs. I have one master processor sending messages, and four slave processors receiving them. I have a fifth slave processor on the comms line which acts as a “comms monitor” dsPIC+LCD and just displays all traffic so I can see what’s going on.

The UART is sending at 100KHz, and I’m using the 9-bit mode, which allows me to ignore data bytes which aren’t intended for a given processor - e.g. each processor only has to keep an eye out for address bytes and check those, rather than having to read every single byte that is sent.

However - I’ve got a problem. Comms to each of the slaves works fine individually, but as soon as I stick a second or further chip in its socket, the comms stops working (Comms monitor registers no messages either) and the slaves do nothing. Currently, it only works with the master, one slave, and the monitor connected.

What’s the maximum “fan out” for a UART like this? Do I need to buffer its output to get more drive or something? Why would connecting more inputs to the signal kill the comms?

Any help or advice appreciated. I’m sure I’m not the first person to have come up against this type of problem…

Thanks,
Tom
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