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To connect our converter to an existing
current loop port, you must first determine if the port is
active or passive. What this means is: does the port have
an internal power supply that provides the current (active)
for the transmitter, the receiver, or both (transmitter and
receiver). The simplest way to determine this is to break
the loop (disconnect it) and see if there is any DC voltage
across the output or input pairs. If you have access to the
instruction manual for the unit you can also look in there
for the information.
Current loop interfaces normally consist
of four wires. They are usually labeled T+, T-, R+, and R-.
T+ and T- are the transmit plus and transmit minus lines and
data is output from that device on those lines. The R+ and
R- lines are the receive plus and receive minus lines and
data is input into that device on these lines. Interconnection
of the two current loop devices is different depending on
whether your unit is active or passive.
3.1 Connection
to an Active Current Loop Port
Connection to an active current loop port is very simple.
Your units T+ and T- lines go to our units R+ and R- lines.
And your units R+ and R- lines go to our units T+ and T- lines.
See the following drawing.
Figure 8. Connection to an Active Current Loop
3.2 Connection
to a Passive Current Loop Port
Connection to a passive current loop port is a little harder.
You must use a 12 VDC power supply with the 470 ohm resistors
inside of our converter to "create" a 20 ma current source.
See the following drawing.
Figure 9. Connection to a Passive Current Loop
3.3 Interconnection
of 2 Current Loop Converters
Interconnection of two B&B current loop converters also requires
the use of a 12 VDC power supply since they are both passive
port. See the following drawing.
Figure 10. Interconnection of Two Current Loop converters
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