2 wire Modbus over 4-20mA current loop

  • Thread starter Itay Weichselbaum
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Itay Weichselbaum

Dear all,

I'm working in a company that develops Level measurement instruments.

I would like to know if there's a way to implement a modbus on a 2 wire 4-20mA current loop?

Does it influence the 4-20mA?

Itay Weichselbaum
HART is a frequency shift keyed audio signal that rides on top of the 4-20mA DC. Analog inputs are generally low pass filtered so the higher frequency HART doesn't influence or bother the DC.

See http://www.hartcomm.org for details.

HART has some means of networking a number of HART devices, but it seems to be rare that any one uses it.

I suppose you could use a technology like that, but you'd have to a supply converter that strips the Modbus packet off the dc signal, puts it on a serial port and passes the DC through.

I have no idea what Foundation Fieldbus runs for a physical layer, whether it's a separate comm port or whether the power for the transmitter is drawn off the comm line.

This is often confusing because Modbus defines the protocol layer and not the physical layer.

HART includes the physical layer in its specification, and specifies Bell202 equivalent signalling and 1200,O,8,1 in their specification.

Modbus over 4-20mA signals is done often. In fact, our instruments use Modbus RS485 and have 4-20mA output with HART on B202. I sometimes assign Modbus over the B202 instead of RS485. Even Micromotion tansmitters do this.

For your application, you need is a Bell202 modem on each end, preferably with auto-driver enables. Set up Modbus for 1200 baud, whatever parity, etc. and you are good to go!

Just like HART, the signal will affect the analog signal somewhat.

FYI: In my opinion, HART is a pathetic excuse for a protocol.