Contemporary Control Systems EXP Expansion Modules User Manual - Manual (Page 18)

+5 V +5 V
Vcm
Rb
Rb Rb
Rb
Rt Rt
TXEN
P1P2
RXIN
TXEN
P1P2
RXIN
Figure 2-2. Each -485 hub port has provisions for applying
bias and termination. Make sure common mode voltage (Vcm)
does not exceed +/–7 Vdc.
Segments of -485 (-485D) connected NIMs can be extended
through the use of active hubs. Select a MOD HUB expansion
module with a -485 compatible port. Connect one end of the
segment to this port following the same termination rules as used
for a NIM. This hub port counts as one NIM when cable
loading is being calculated. The NIM electrically closest to the
hub port should not have any termination or bias applied.
Follow the same rules for other segments attached to different
hub ports. Each hub effectively extends the segment another 900
feet. Maintain the same cabling polarity as the NIMs by using
cable connections that do not invert the signals.
2.5.7 Connecting AC-Coupled EIA-485 Networks
The AC-coupled EIA-485 transceiver offers advantages over the
DC-coupled EIA-485. No bias adjustments need to be made
since each transceiver has its own fixed bias network isolated by
a pulse transformer. Unlike the DC-coupled EIA-485, wiring
polarity is unimportant. Either inverted or straight through cable
can be used or even mixed within one AC-coupled network.
Much higher common mode voltage levels can be achieved with
TD401600-0M
O
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