Post by thread:Multi-drop serial bus

> Hello, > > There was a thread on the list a while back about implementing a > multi-drop serial bus for PIC communications (RS422, I believe). Can > anyone help me find it? I would like to know the resolution, ie, did > they find a way to do it? Is there existing code to implement this bus? > Any help appreciated. > > Thanks, > > Martin Kirk > Arizona State University > spam_OUTmlkTakeThisOuTasu.edu > (602) 263-9270 Martin, I don't remember the discussion but let me explain something that works quite well for me at low data rates (1200 baud). I haven't hooked it to a scope to see the waveforms but I THINK the design is sound. What I do is tie the Rx line going to the computer to RTS through a 2.2K resistor. I write my programs to pull RTS low (-12V). I then connect the Rx line through an opto-isolated transistor to +12V. I can then switch the line from -12V to +12V by energizing the optoisolator. Also, I can connect many of these together this way. Then, I just connect the Tx pin of the computer to all my PICs through a large resistor (I think I used 1Mohms so there was very little loading). Now, you have to work on your protocol because collisions can definately take place. Also, a limitation is that you cannot have one PIC talk to another - just to the computer. If anyone has any comments on this, I'd like to hear them. I've been using this method for a long time - with no problems at all. -- -- Paul Greenwood -- (.....pabloKILLspam@spam@austin.ibm.com) "We don't care. We don't have to. We're the Phone Company."