Is the RS485 Interface RS485 'Spec' or RS485 TTL?

Backgroud:  We have a radio that talks RS232/RS485/RS485HD/RS222.  The radio has it's own power supply of 12 V.  The radio communicates fine using RS232 with an RS232 to RS485 converter with the Industruino device.  NOTE that I am using the 1286 top board and the IND I/O board.  Also note that we are using Modbus to communicate from a Modbus master software on a computer to the Industruino device and the device is responding to a 03 (Get holding register) call.  When we try to communicate via RS485 directly to the device, the query gets to the device and the device is responding but it never gets back to the modbus master.  Remember that this scenario works fine with RS232 converter.  So, what we have been told, or the theory, is that because the radio is 12 V and the Industruino device is 5V (two power supplies), the difference in the ground differential is too much and is why the signal never makes it back to the caller.  Thus, my question is:  Is the RS485 interface in the Industruino IND I/O board RS485 'Spec' or RS485 TTL?  Note that TTL is lower voltage 485.  I suspect the answer is that it is RS485 but if you can give me another path I would love to look at it.

P.S.  Note also that we tried it with and without the bias jumpers on the I/O board.

Thanks,

Steve

 

Steve May
Steve May
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Asked on 7/14/16, 6:23 PM
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Stefano,

      Thank you for the response.  I have used the device with other port servers such as a Lantronix Ethernet to serial device and it worked just fine.  It sounds like what you are saying is that I would have to connect, through a resister from a 12V power supply to one of the bias resisters on the board.  That may actually work because it would equalize the bias, however, that is not really a viable solution.  We don't want the technicians in the field to have to do delicate work like that.  I will try them on the same power supply which I have not done yet and see if that makes a difference.  I will let you know what I find.

Steve

Steve May
Steve May
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Answered on 7/20/16, 2:02 PM
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Hello Steve, what I say is that this is not an issue related to Arduino, rather than to RS485 topology. I'm not a great professional in that, but I think there should be only one bias over the rs485 network. You could try and inject +12 via a pullup resistor over a separate external power supply and see what happens. I would also add a 200 ohm resistor in series with the Industruino RS485, just in case! You can build an intermediate connection unit between the Industruino and your radio with all these item inside.

Stefano Giuseppe Bonvini
on 7/21/16, 12:20 PM

Hello,

as far as I recall, the chip on Industruino is MAX485.

You should check this on the chip documentation, but probably all you need is a pull-up resistor to 12VDC in order to make it work.

Being RS485 a daisy chain parallel standard, outputs should be tri-state pull-down type.

So a pull-up should do the trick, but check on the datasheet first!

Stefano Giuseppe Bonvini
Stefano Giuseppe Bonvini
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Answered on 7/20/16, 8:00 AM
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Asked: 7/14/16, 6:23 PM
Seen: 923 times
Last updated: 7/20/16, 2:02 PM