Technical data

32 Theoretical and general applications www.westermo.com

10 mA balanced current loop (W1)

Westermo has developed its own transmission technology for short-haul modems

that ensures communications over greater distances and in environments with a high

level of interference. The technology is based on converting the signals to a ±10 mA

balanced current loop, where the current direction is shifted on the wire pair, depend-

ing on whether it is a high or low signal from RS-232/V.24. The line on the transmitter

is powered by ±10 mA and an optocoupler is fitted on the receiver to detect the sig-

nals. The optocouplers provide complete galvanic isolation between modems. Current

is always flowing in one direction even when there is no equipment connected on the

RS-232/V.24 side. The exception is when the transmitter is controlled/activated using a

handshaking signal. It is a tried and trusted technique that over the years has proven to

be very reliable and insensitive to interference and supports data transmission at

ranges up to 18 km (11 mi).

Consequently the 10 mA balanced current loop is less sensitive to exter-

nal sources of interference.

Compared with an unbalanced current loop, a balanced current loop is significantly

less sensitive to external disturbance due to the potential differences remaining even

when interference is experienced on the line. See the figure below.

1. Data is sent to the

transmitter.

2. Data on wire A is inverted

compared to the data on

wire B.

3. The line is exposed

to interference.

4. Transmission data super-

posed on the interference.

5. Data arriving on the receiving

side is unchanged from the

data sent by the transmitter

(1).

Wire A

Wire B

B A C K