The RS485 converter that the manufacturer sells has 4 signal wires going into 3 NPN and 2 PNP DTR's, then going into the RO, RE, DE, and DI pins of an isolated RS485 transceiver (LTC1535ISW). That is working as it is supposed to. When I remove the LTC1535ISW from the board, and wire the RO, RE, DE, and DI signals to a standard MAX485 type IC (ST485BN, in this case), I can't get it to work. I can't figure out where the discrepancy is between the
ST485BN and the LTC1535ISW that keeps the former from working properly, when everything else is the same. The only thing missing is the isolation circuit to the RS485 side. My ultimate goal is the connect the RO and DI signals directly to another device that uses TTL once I can figure out what is going on. I am reverse engineering.