Choosing Module Features
Chapter 3
33
flow. A receiving device in the loop senses the interruptions. By
convention, a logic 1 corresponds to the presence of loop current; a logic
0 corresponds to the absence of loop current.
A current loop transmitter or receiver can be either of two types: active
(source) or passive (sink). An active transmitter supplies current to the
loop. Any receivers or other transmitters within that loop must be passive
units that accept the supplied loop current. Alternately, an active receiver
supplies current to passive transmitters or other passive receivers in the
loop.
Current sources that power a current loop vary in complexity. The
simplest is a resistor and voltage source. More complex current sources
contain active elements or integrated circuits to provide constant current
under various power supply and load conditions.
Refer to Table 2.B and Table 2.C for a detailed listing of current loop pin
functions.
Table 2.B
Current Loop Connector Pin Functions
Passive Receive/Passive Transmit
Pin No.
Signal Name
Source
Function
11
Module Transmitter Circuit
Peripheral or Power Supply
Controls current loop, allowing
peripheral device to read data
12
Module Receiver Circuit
Peripheral Device
Completes current loop, allowing
transfer of data to 1771DA
18
Module Transmitter Circuit Return
Return for module transmitter circuit
24
Module Receiver Circuit Return
Return for module receiver circuit
Table 2.C
Current Loop Connector Pin Functions
Passive Receive/Active Transmit
Pin No.
Signal Name
Source
Function
11
Module Transmitter, Circuit
Control and Return
Controls current loop, allowing
peripheral device to read data. Serves
as return for transmitter circuit.
12
Module Receiver Circuit
Peripheral Device
Completes current loop, allowing
transfer of data to 1771DA
13
Module Transmitter Circuit Source
1771DA
Supplies current for current loop
interface
24
Module Receiver Circuit Return
Return for module receiver circuit
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