Daisy Chaining HSER RX ports

[Charles Linquis]

I guess I should add - the reason you receive in hardware and send in software is -
Hardware receive is the only way you can never miss a byte (and still have your program doing something).
Software send is the only way you can get an open-collector output without using an extra chip on your board.

[ardhuru]

Your scheme should work well. As macrackit says, I would use a transistor at the master's tx, with a pull-up to +12V. Then fan out the output to all the slaves.

Did something similar, but it was the other way. All slaves *sending* to a single pin of the master pic. In this case, I had to use a rudimentary scheme to cater for the collisions.

Regards,

Anand

[dhouston]

You might also consider using a DG408 or MAX398 multiplexer IC (with or without a RS232 transceiver) which doesn't require as much fancy footwork. You can find app notes using it for a pseudo-multidrop RS232 setup. However, it does require a few extra pins.

[markscotford]

Thanks everybody, thought I may have run before walking, but seem to have got away with it. Thank you Charles, your post was very helpfull. The boards are all daisy chained with 10 way ribbon cable, linking Power & HSER ports together (with some input protection). The boards are no more than 10cm away from each other. The hardware receiving, software sending idea, switching off modes between communications idea is also helpfull.

Let the heart aches and long nights begin.

[Charles Linquis]

Believe it or not, I used the technique described to communicate over 340' of Belden 24GA unshielded 4 conductor cable (power, ground, transmit, receive).
I was running 9600 baud and I had to reduce the pull-up resistors (1.5K ohms on each board), but it worked. No signal boosting - nothing but two 18F2221's on one end and an 18F8723 on the other.

[Charles Linquis]

Also, the 2221's were running on their internal oscillators, the 8723 was on a crystal.