Michael,

Thanks for these useful guidelines. I can see the benefit of the "receive always on" approach that you suggest - however I don't want to change the hardware if I can avoid it because there are hundreds of these systems out in the field. So I'd like to come up with the most reliable, bullet-proof way of driving the bus with /RE and DE connected together and to my port pin. Here's what I've got at present (this just echoes all characters received):

• At the beginning of the loop I can't be sure what state everything is in, and other code may have been using the RS485 bus.
  
• turn driver off and receiver on, because I'm not sure what state it's been left in
  
• wait a moment for bus settling etc.
  
• do I need to read SBUF to make sure the chip is ready to receive the next byte?
  
• do I need to clear RI here in case something has been received when I switched the receiver off and on again or by "other code"?
  
• jnb RI,$
  
• clr RI
  
• mov a,SBUF
  
• wait half a bit-time plus a moment for settling
  
• turn driver on and receiver off
  
• wait a moment for settling
  
• do I need to clear TI here, just to be sure?
  
• mov SBUF,a
  
• jnb TI,$
  
• clr TI
  
• wait 1 bit-time plus a moment more for bus settling etc.
  
• turn driver off and receiver on
  
• wait a moment for bus settling etc.
  
• "other code" which may receive and/or transmit via the RS485 port
  
• repeat the above in a loop

Can you suggest any improvements to this?

Yes, I know that the best way to do it is with interrupts! I will move on to that when I've got this working perfectly...

Thanks - Rowan