How to determine 485 network is not busy in half-duplex

[Jerson]

I have done multi-master RS485 many years back. I found the AN01230 interesting in how they achieve the multimaster operation.

What I did involved a little an extra 485 chip, but, essentially utilised the same principles.

I had a main RS485 transmitter and a separate RS485 listener chip on the same board. Whatever goes out the transmit line is visible back again on the input line via the listener chip. So, any data corruption due to a collision can be detected and the transmit process can be re-started. The rest of the arbitration process is essentially the same except I use discrete IDs for each RS485 station like in any such multi-device setup and the system worked very well.

You can stop transmitting the moment you see a single byte error in your packet - when sent character is not same as received character.

[ecoli-557]

Thanks Jerson. I made the mistake of already having the board made so I would like to try to see if I can do it with just one 485 transceiver. The J1708 looks like it is still being used and if I cannot get a modified J1708 protocol to work with a 485 connected the normal way, then my next try would be to try an actual J1708 transceiver. Maxxim makes one and it look as if it would be a drop-in replacement so I would have the expense of re-doing the board design.
I think I have base code working but for some reason the 485 enable line sits high - even after the packet should have been delivered..... which tells me the program thinks there is still stuff to transmit...... looking into it now....
Regards,
Steve

Code below:

Node_ErrChk:'-------------------------------------------------------------------
'Checks the received node packet for data integrity and sends an ACK if good or a NAK if bad
                  NodeWord=Node_Addr+Node_Cmd_Resp+Node_Bd+Node_Port_Dev
                  Node_Sum_temp=NodeWord.byte0 ^ $FF                            'Creates a local temp sumcheck used for comparison
                  if Node_Sum_Temp <> Node_Sum then                             'If the received checksum and the local checksum don't match, then.... 
                     error=1                                                    'Turns ON error LED
                     Node_Cmd_Resp=$03                                          'Form our NAK response
                     Node_Addr=Address                                          'What our local address is
                     Node_Bd=0                                                  'Doesn't matter, we are asking for a retransmit
                     Node_Port_Dev=0                                            'Same as above
                     NodeWord=Node_Addr+Node_Cmd_Resp+Node_Bd+Node_Port_Dev     'Create our checksum
                     Node_Sum=NodeWord.BYTE0 ^ $FF                              'ditto
                     ncts=1                                                     'Enable transmission of 485 interface
NodeLineBusy2:       if BAUDCON1.6=1 then pauseus (Address*10)                  'Checks to see if the receive operation is idle(1), if it is, pause for address*10 uS for random
                     if BAUDCON1.6=0 then NodeLineBusy2                         'Receive operation is still busy, if not, send packet
                     hserout [sync,Node_Addr,Node_Cmd_Resp,Node_Bd,Node_Port_Dev,Node_Sum]   'Send command
                     while not TXSTA1.1 : wend                                  'Wait for USART to finish xmitting
                     ncts=0                                                     'Places 485 interface back into receive mode
                  else                                                          'OR....... 
                     Status1=1                                                  'Turns ON LED
                     NodePktResp=Node_Cmd_Resp & $0F                            'Strips off high order nibble to check for a NAK, put in local temp var - keeps packet integrity
                     if NodePktResp=$03 then goto Node_ReXmit                   'Got a NAK from the Control Computer so send it again
                     if NodePktResp=$02 then Node_ErrChk_Exit                   'We were ACKd, so just leave it alone
'Now we have received a node packet, believed to be good, so we will ADD $02 to 
'the Node_Cmd_Resp byte we received and transmit back to Control PC so the PC 
'knows everything is OK in our world....... and figure a new Checksum......
                     Node_Cmd_Resp=Node_Cmd_Resp + $02                          'Adds the ACK to the byte for our response
                     NodeWord=Node_Addr+Node_Cmd_Resp+Node_Bd+Node_Port_Dev     'Build our new checksum
                     Node_Sum=NodeWord.BYTE0 ^ $FF                              'ditto
                     ncts=1                                                     'Enable transmission of 485 interface
NodeLineBusy3:       if BAUDCON1.6=1 then pauseus (Address*10)                  'Checks to see if the receive operation is idle(1), if it is, pause for address*10 uS for random
                     if BAUDCON1.6=0 then NodeLineBusy3                         'Receive operation is still busy, if not, send packet
                     hserout [sync,Node_Addr,Node_Cmd_Resp,Node_Bd,Node_Port_Dev,Node_Sum]   'Send command
                     while not TXSTA1.1 : wend                                  'Wait for USART to finish xmitting
                     ncts=0                                                     'Places 485 interface back into receive mode
                  endif
Node_ErrChk_Exit:
                  Status1=0                                                     'Turns OFF LED
                  Error=0                                                       'Turns OFF LED if it was ON
                  node_PacketRcvd=0                                             'Zero the switch so we don't send other bytes
                  return