Black Box ME540A-ST, Manual, Page: 17 / 26

FOM LINE DRIVER—ST AND S M
1 6
Figure 5-3. Circuit board with 20-mA current-loop options.
+ +
+
+
+
-
+
+
+
+
+
+ B
- A
+ B
- A
Rx
Tx
1
2
3
4
RS-232
A
S3 S2
S1
A
B B
A
B C
B
A
RX B
RX A
TX B
TX A
RX A
TX A
RX B
TX B
Slave
Master
Network
20mA
Loopback
Switch
RS-232
RS-485
20-mA
Terminal Block
S5 set to
A (20mA)
XW2C = 20-mA
interface
S2 and S3
S2 Pos. A = Active RX
S2 Pos. B = Passive RX
S3 Pos. A = Active TX
S3 Pos. B = Passive TX
S1
Pos. 1 = Mode Select
0 = Network Mode
1 = Master/Slave Mode
Pos. 2 = Master/Slave Select
0 = Master
1 = Slave
KEY
0 = Open/Off
1 = Closed/On