My new customer is saying that the unit is sometimes not responding to the sync command (0x80). The unit supposed to replay with 2 informations where ever the master issue sync (0x80) command. But sometime the master need to issue 2/3 sync command to get a reply. They are using 1-to-1 communication. The unit uses AT89C51CC01 micro with opto-isolate HCPL 710) output. Anybody could suggest where to start in my firmware, or, is this could be a hardware issue?