ON 20010802@11:14:20 AM at page:
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cd9985-bris-5EA    added 'Questions:

Dear All,

I'm developing a simple shared bus with multiple PIC based devices communicating using RS232.  The RS232 stuff is working fine all I need to know is how to build a line to resolve contention issues.

My idea was to have a single wire that connects to 2 pins on every PIC device with only one end of the wire going to ground via a 10K resistor.  One of the pins on the PIC is set to input (B6) and the other is set to output (B7).

When a PIC wants to transmit, it checks whether B6 is clear (wire is at 0V >> idle state) and then uses B7 to set the wire high (+5V >> hence has control of bus).  The device then transmits data on different pins (port A). When it has finished transmitting data the PIC then clears wire to 0V using B7.

This is ok in theory but in practice it is intermittent >> sometimes instead of a PIC pulling a 0v line high to +5V it pulls it to +0.7V and then won't pull it back down to 0V until another device pulls it correctly to +5V.

I think I need some diodes and some shunting resistors but I don't remember my electronics studies well enough (been a mad programmer for too long!!)

If anyone has any ideas I would really appreciate your help and you would be mentioned as a contributor to my PhD in mobile and wearable computing.

With Thanks,

Chris Djiallis
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