Black Box ME540A-ST, Manual, Page: 13 / 26

FOM LINE DRIVER—ST AND S M
1 2
5.
Biased, Failsafe Operation (S1, positions 3 through 6).
S1 positions 3 through 6, when in the 1 or
ON position, switch in a 1K pull-up resistor to the A input/output and a 1K pull-down to the B
input/output (see Figure 4-1). 0 is the OFF position.
In almost all 2- or 4-wire RS-485 applications, positions 3 and 4 should be OFF and positions 5 and 6
should be ON. Note that positions 3 and 4 and positions 5 and 6 should always be moved in pairs,
either both open or both closed.
NOTE
To use S1 positions 3 through 6 appropriately, you should have a firm understanding of
your network requirements.
Figure 4-1. Biased/Failsafe Operation Switches (RS-485).
Data Flow
TB1-4
TB1-1
TB1-2
TB1-3
+5V
+5V
1K
TX
RX
1K
1K
1K
Line Driver
S1 Position 3
S1 Position 4
S1 Position 5
S1 Position 6
Data Flow