Modbus ASCII to Modbus RTU Protocol Converter


Thread Starter

Mark Mullins

I have a SCAD system talking Modbus RTU and a battery charger system talking Modbus ASCII. Does any have any information on protocol converters that will enable me to connect these battery chargers.
I'm sure someone already covered the "MB/RTU in TCP has the CRC16 and MB/TCP has a 6 special byte header and assumes the CRC32 in TCP". But if you are using Wide-Area-Network technology, a HUGE difference is Modbus/RTU in TCP has no sequence number (TCP does, but that's a different topic). Modbus/TCP includes a sequence number - which most apps should respect (a few sadly ignore). This means if send a MB/RTU func #3 read registers, don't get an answer for a long time and send a second try, when you do get the answer you cannot know which of the 2 polls the response if for (ie: you cannot be sure a 2nd response is not on the way back also). This can result in some application confusion. This causes me endless headaches - Ethernet is *NOT* RS-485, yet most Modbus/TCP applications try to treat TCP/IP like RS-485 2-wire (assume if you see no answer, it's not there). This ignores the fact that the response may be "in the mail" if you have a satellite system with 700-1200 msec delivery delays. Whenever possible, chose the Modbus/TCP and insure your applications respect the sequence number. regards Lynn August Linse, Senior Software Engineer 15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618 [email protected] Tel: (949)300-6337 Fax: (949)453-7152