Contemporary Control Systems AI Active Hubs - Manual (Page 22)

TD675100-0M
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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.
2.6.6 Connecting AC Coupled EIA-485 Networks (-485X)
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 AC coupling due to the transformer coupling which has a
1000 Vdc breakdown rating.
There are disadvantages to the AC coupled transceiver as
compared to the DC coupled technology. The DC coupled
distances are longer (900 feet) compared to the AC coupled
distance (700 feet) and the node count is higher with DC. The
AC coupled transceiver will only operate between 1.25 Mbps
and 10 Mbps.
The cabling rules of the -485X are similar to the -485. Wire a
maximum of 13 NIMs (reduce by one for each AI port) in a
daisy-chain fashion leaving the end devices as either NIMs or
AI ports. On these NIMs or AI ports insert a jumper at E1 on
both -485X daughterboards to invoke 120 ohm termination
resistors or leave the jumpers open and connect an external
terminating resistor to phases A and B. Termination should not
be applied to any of the NIMs located between the two end
NIMs or AI ports of the segment. Mixing -485 and -485D can
be accomplished by invoking backplane mode on -485 NIMs
and non-backplane mode on -485D NIMs.