Black Box ME540A-ST, Manual, Page: 16 / 26

FOM LINE DRIVER—ST AND S M
1 5
Figure 5-2. Circuit board with RS-485 options.
+ +
+
+
+
-
+
+
+
+
+
+ B
- A
+ B
- A
Rx
Tx
1
2
3
4
RX A
TX A
TX B
A
S5
S1
W6
W5
W4
B
B
A C
RX B
Slave
Master
Network
20mA
RS-485
RS-232
Loopback
Switch
RX A
RX B
TX A
TX B
KEY
0 = Open/Off
1 = Closed/On
W4 and W5
Set for the AB position = 4 wire
Set for the BC position = 2 wire
W6
When in 2-wire, this is the time
that the driver remains enabled
after the TX of the last data bit.
Delays are:
A = 7 ms
B = 2 ms
C = 0.7 ms
D = 0.15 ms
XW2B = RS-485
interface
S5 set to
B (RS-485)
RS-485
Terminal Block
S1
Pos. 1 = Mode Select
0 = Network Mode
1 = Master/Slave Mode
Pos. 2 = Master/Slave Select
0 = Master
1 = Slave
Pos. 3 & 4 = Bias TX (see pg. 9)
0 = No Bias
1 = Biased
Pos. 5 & 6 = Bias RX (see pg. 9)
0 = No Bias
1 = Biased
Pos. 7 & 8 = RS-485 Termination
0 = Unterminated
1 = Terminated