Ok, to clarify a few things about RS-485/modbus. RS-485 is a multidrop, balanced, async serial bus, designed to run for fairly long distances over a single 120 ohms twisted pair. General you're going to have A and B which are the data lines (complimentary), and some sort of third wire which is reference (labeled something like Common, or GND, or something else). If it's RS-485 there are various RS-485 to serial USB devices out there, some isolated. If I was told that it *was* RS-485, I would suspect the two +5 low current lines are actually A and B, and the +5 and GND are there to power the cable and act as the 'C' wire as well.
Modbus is just a serial protocol which runs on top of a serial bus, typically RS-485. It's usually RTU which means binary. It's a pain in the rear to write since as someone else mentioned, there are timing commitments to make, although it isn't as fussy as one would expect from reading the spec. There are also typically standardized values you can read to get the manufacturer. However, the protocol itself doesn't carry any data about what the actual data values are or where they can be found. This needs to come in the form of documentation from the vendor. On Wed, Aug 19, 2020 at 10:27 AM <ch...@wbmfg.com> wrote: > On one of my large portable air compressors, there is a data port. They > sell an adapter cable for about $200 It adapts it to USB. The program > that you use to talk to it makes reference to modbus. > > The data port has 4 pins. RJ-10 4p4c connector. > +5 with enough current to light an incandescent light bulb. > +5 low current > +5 low current > Ground > > This sounds like USB host to me. One of my sons think it is RS-485 > > I don’t think USB would be happy on a RJ-10 and utp wire. Maybe it > would. Maybe I am just buying a piece of wire with two connectors on the > ends. > > The controller it plugs into is $2000 so I don’t want to experiment on > that. Be interesting to see the adapter cable when it gets here. See if > there are any active components. I would have probably just wired it up to > a USB and tried it if my son had not made the modbus/RS485 comment. > > Any guesses? > -- > AF mailing list > AF@af.afmug.com > http://af.afmug.com/mailman/listinfo/af_af.afmug.com > -- - Forrest
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