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FAQ - Single Direction Current Loop

 

This FAQ covers Single Direction use of current loop with transmitting devices. 
Q:  How do I connect the Current Loop Converter to my Current Loop device?

Transmit and Receive Loop

 

 

A:   This Current Loop interface has two states, during the Mark state (no signal), current is 
flowing in each loop, transmit and receive.  In the Space state, no current flows.  Open circuit, 
an Active Current loop will have a DC voltage, a Passive loop will have no voltage.   

Use a DC voltmeter to measure between T+ and T- to see if a DC voltage is present when 
the device is turned on, but T+ and T- are open circuit (not connected to anything but the 
voltmeter).  If a DC voltage is present (FIG.1) , it will generally be more than 1.2V DC, open 
circuit may be +5, +12 or more (depending on the Current Source Voltage, but should current 
limited to around 20mA closed circuit.  If the T+ to T- wire pair has voltage, the converter 
Receive must be connected as a Passive device.  If there is no voltage, the converter 
Receiver must be made Active.  Measure and determine whether the device R+ and R- pair 
is active, if active, the converter Transmitter should be passive.   If Receiver is Passive 
(FIG.2), the Transmit on the converter must be made Active. 
 
In the figures which follow, the converter loops are passive, or active or one of each, similar 
wiring is shown for each of the 3 models.  Download the data sheets for internal diagrams. 

232CL4R Passive Converter


232PCLR Passive Converter - Active Device
232CL4R Active Converter - Passive Device

To maintain isolation between the RS-232 device and the Current Loop device, a separate 
isolated power supply should be used to make the Current Loops active.  If the Current Loop 
is made active using the same supply as the RS232 side of the converter, there will be no 
protective isolation. 

232CL9R Active Converter - Passive Device
Active Converter - Passive Device

To maintain isolation between the RS-232 device and the Current Loop device, a separate 
isolated power supply should be used to make the Current Loops active.  If the Current Loop is 
made active using the same supply as the RS232 side of the converter, there will be no protective 
isolation. 
 
When not sending or receiving data, each Current Loop should have around 20mA of current 
flowing.  This can be measured for each loop by connecting a meter with a current scale in series 
with the Transmit or Receive line, observing meter polarity.  In closed loop mode, the voltage 
drop across the Transmitter will be minimal, depending on the circuits, about 1.8V at the 
transmitter, around 1.8V at the Receiver. 

DTE to DCE

 

The RS-232 Connections depend on the type of Device the converter is connected to on the RS-
232 side, if it is a DTE or DCE device.  Computer devices are usually DTE, Data Terminal 
Equipment, Modem devices are DCE.  Transmit is an output from a DTE, Receive is an output 
from a DCE.  Devices wired the same must swap the connections for TD and RD.

Checking Pins to Determine:  How to identify which of two lines is the output from the device:
 
Use a DC voltmeter, measure from the ground pin to Pin #2 or Pin#3.  Usually one Pin from the 
source device has a minus (-) DC voltage, usually between –11 volts and –3 volts.  Whichever 
lead has a minus voltage should be connected to the TD input pin, Pin #2 on the DB25, (Pin #9 
on the 232CL9R)  The other pin usually has no voltage or just noise relative to ground.  If neither 
lead has a minus (or positive) voltage on it relative to ground, recheck for OPEN cable 
connections to the RS-232 device or the device pinouts.  If the RS-232 device can be configured 
multiple ways, make sure all the jumpers and such are set to RS-232.   

A few devices may use very low power RS-232 ports which switch only between Ground and 
positive voltage, so to identify which is line is active, the device must be set to transmit data, then 
tried. Some DC meters may show a slight flicker of DC or AC voltage on a RS-232 data line with 
changing data. 
 
If the Current Loop Device is only sending data (such as a scale), and the Current Loop converter 
is only receiving, the Transmit data from the device only needs to connect to the Receive inputs 
on the Current Loop Converter, and Ground and Receive from the Converter to the computer or 
scale display. 
 
This information should help you make the needed connections.