FAQ: How do I connect my Digital Current Loop or RS-232 device to the 232CLDR

How do I connect my RS-232 device to the 232CLDR?

232CLDR Figure 1


First determine if the Current Loops for the device are Active or Passive. 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 matching 232CLDR Receiver must be connected as a Passive device (SW2=OFF). If there is no voltage, the 232CLDR Receiver should be made Active (SW2=ON). Measure and determine whether the device R+ and R- pair is active, if active, the 232CLDR Transmitter should be passive (SW1=OFF). If Receiver is Passive (FIG.2), the Transmit pair on the 232CLDR must be made Active (SW1=ON).

Figure 3

Fig 4 Diagram

Fig 5 - Diagram

A2:The RS-232 Connections depends on the type of device the 232CLDR is connecting to, if it is a (1) DTE or (2) DCE device. Digital Current Loop converters support only Receive (Rx) and Transmit (Tx) and use a “3-wire” RS-232 interface, Receive (Rx), Transmit (Tx) and Ground.

A: See example figures below:

232CLDR Connections to DTE and DCE

  1. The RS-232 device on the left side (Figure 6) has pinouts which are DTE and match a computer with DB9M connector. Pin 3 is the output labeled Tx, pin 2 is the input, pin 5 is ground. DB25 Pinouts are shown next to DB9 on the next page.
  2. The RS-232 device on the right side (Figure 7) has pinouts which are DCE and match a modem, so pin 3 on the device is input, pin 2 is output, 5 is ground. For DB25 pinouts, see figures on the next page.

How to identify which line from the RS-232 device should connect to Terminal D

Using a DC voltmeter, measure from the ground wire to each of the other two RS-232 wires, while the RS-232 device is powered up. Usually one lead has a minus (-) DC voltage, typically between –11 volts and –3 volts. Whichever lead has a minus voltage is the lead to connect to the 232CLDR Terminal D. The other lead 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, so to identify which is line is active, the device must be set to transmit data, then connect the ground wire to 232CLDR terminal B and one of the wires to D, see if the data indicator pointing to the Curent Loop side flashes. If not, try the other wire. If the data request is coming from the other end, and all that wiring is correct, the indicator pointing toward the RS-232 side should be flashing. Some DC meters may show a slight flicker of DC or AC voltage on a RS-232 data line with changing data.

If the LED pointing toward the RS-232 side is lit all the time, the Current loop to the Receive input is Open or has another fault. It should be off when the Receive Loop is Active, except during data.

232CLDR DTE Connections

DCE Connections

If the Current Loop Device is only sending data (such as a scale), and the 232CLDR is only receiving it, 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 is provided to help you make the needed connections.