Specifications

42
Schneider Electric
20 mA CL – RS232 – RS422 –
RS485 links (continued)
The 20 mA current loop
A 20 mA current obtained from the power supply voltage (12 V or 24 V) flows through
the loop.
For normal lines (20 mA: resting state on the line) approximately 1.5 V is lost for each
station connected.
b Transmitters for slave stations are usually connected in series with the receivers of
the master station (normal line, 20 mA at rest):
Master Slave Slave Slave
b Transmitters for slave stations can be connected in parallel with the receivers of
the master station. In this case the resting state of the line is 0 mA:
Master Slave Slave Slave
This transmission mode, used on some
equipment, provides better performance
than RS232C mode. Although this mode
provides good immunity to parasites, it is
not always the best solution because the
current loop is not standardised.
General characteristics:
b Maximum number of loads on the bus:
depends on the type of transmitters/
receivers.
b Standard topology: "Multi-point" of the 4-
wire bus type.
b Transmission speed: 300/600/1200 Bds.
b Electrical levels: 0 mA / 20 mA.
b Control lines: None.
b Length of the bus: 3000 m from 300 to
1200 Bds. Depends directly on type of
cable and voltage of bus power supply (12
V or 24 V).
E52338E52339
-
+
+
+
-
+
+
+
+
+
-
-
-
-
-
-
T
R
TD R R
R
RD
T
T
T
TD
TD TD
-
+
+
+
-
+
+
+
+
+
-
--
T
R
TD R R
R
RD
T
T
T
TD TD TD
Notes:
b The sign "+" is used for incoming current
and "-" for outgoing current.
b In the figure, T = transmitter,
R = receiver .
Guide to Integration
Modbus / Jbus