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 On 4/4/2016 12:52 AM, Nicklas Karlsson wrote: >>>>> ISO7421 >>>> Now thats sweet, and a heck of a lot better thought out than the >>>> last such chip I looked at a decade ago. Needs a 4 wire cable from >>>> each direction, but I don't see as that as a problem other than >>>> stealing the ground and 3.3 or 5 volts to run its side of it at >>>> both ends. >>>> >>>> That should indeed sove the noise problem. The pcb requires a >>>> slightly different layout of putting the power on the center layers, >>>> so the best bet is to look around and see if someone might have it >>>> all boxed up and ready to connect. >>>> >>>> Were you able to find such a ready-made critter? If so where? >>> No I made my own circuit board, two layer. SO footprint should not be >>> to hard to solder there are small prototype boards or similar on >>> Farnell. >>> >>> http://se.farnell.com/roth-elektronik/re932-01/pcb-adaptor-smd-so-8-20 >>> -5mmx8mm/dp/1426169 >>> http://se.farnell.com/roth-elektronik/re932-03/adaptor-smd-so-14-1-27m >>> m/dp/1426171 >>> http://se.farnell.com/roth-elektronik/re932-02/adaptor-smd-so-8w-1-27m >>> m/dp/1426170 >>> http://se.farnell.com/roth-elektronik/re932-01st/multi-adapter-11-5x16 >>> mm-soic-8/dp/2292022 >>> >>> I do not have time to check if footprint is correct. >>> >> Immaterial as you wouls wire to suit, but all of them are missing a place >> to put the recommended supply rail bypassing. And extra plated thru-hole >> in each runner to the terminal would be nice. >> >> But who is Farnell on this "west side" of the pond? Or are they even >> affiliated with anybody in the US? > Maybe it is newark http://www.newark.com/ otherwise I think digikey may be > closer to you. > > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
