adccommunitymod (AutomationDirect) asked a question.
RS485 Modbus Slave with 250 CPU
Created Date: October 19,2001
Created By: DJE Design
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It seems to me that the bottom port of the 250 CPU could be used on a 3 wire Modbus system without utilizing an RS232 to RS485 converter (or 422 to 485). You would, as in remote I/O, wire TXD+ to RXD+ and TXD- to RXD-. If the 250 were a Modbus slave, the transmitter would necessarily have to Tri-State (high-impedence) except when responding. Has anyone tried this?
Created Date: October 21,2001
Created by: dcharv
I have tried but it didn't work http://forum1.automationdirect.com/board/frown.gif
I ended up using an F2-UNICON and it worked fine http://forum1.automationdirect.com/board/smile.gif
Created Date: October 24,2001
Created by: Mike McClanahan
The port is Modbus Protocol. Modbus is not a hardware standard but is a communications protocol specification. The port on the 250 is still following the RS-232 standard which is +/- 15V for on and off, not high impedance. Besides why risk damaging the communications port and having to replace the CPU because of a ground loop current problem from a power hit when an inexpensive RS-485 converter solves the problem and can provide some protection at the same time.
Mike
Created Date: October 19,2001
Created by: DJE Design
It seems to me that the bottom port of the 250 CPU could be used on a 3 wire Modbus system without utilizing an RS232 to RS485 converter (or 422 to 485).
You would, as in remote I/O, wire TXD+ to RXD+ and TXD- to RXD-. If the 250 were a Modbus slave, the transmitter would necessarily have to Tri-State (high-impedence) except when responding.
Has anyone tried this?