> That's a bare unidirectional isolator. OK for RS232. Problem being, > Modbus is over RS422/RS485, a bidirectional protocol and there's a lot > of difficult problems in creating buffers of any sort. It doesn't know > which direction it's supposed to drive at any given time. > > http://www.mouser.com/Search/Refine.aspx?Keyword=rs422+isolator&Ns=Pricing%7c0&FS=True > > There are isolating RS422/RS485 transceivers. However, as you can see, > it starts from a 4-wire R/RE_n D/DE logic-level interface, like any > transceiver. That device can't take in RS485 on both sides. > > I used a high-tech ADAM-4520, which takes in RS232 and a DC power on one > side, produces galvanic isolation for RS422/RS485 and isolated power to > drive that logic. Not that expensive on eBay. It's gotta be > protocol-aware to do this, so you MUST set the baud rate and format via > DIP switches and follow the RS485 signaling protocol. > > That is, if you just followed the 8N1 9600 baud RS232 format and sent > your own generic RS232 bytes, it wouldn't know which direction to go, it > needs a Slave Address/Function Code/Byte Count/etc bytes of an RS485 > message. Then, knowing how RS485 protocol works, changes bus direction > as required. > > Both Mach3 and LinuxCNC WILL command a serial port with proper RS485 > messages, even though they're bytes and may be on an RS232 bus (or > logic-level 8N1 9600 baud serial). But I don't know how you'd get > LinuxCNC to produce the raw 4-wire R/RE_n D/DE interface for an RS485 > transceiver. > > Danny
I think the RTS signal is common but it must of course also be insulated. http://www.moxa.com/resource_file/857220091121341.pdf Nicklas Karlsson <[email protected]> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
