TD040900-0I
E
6
-4000 AC-Coupled EIA-485
The AC-coupled EIA-485 transceiver offers advantages over the DC-
coupled version. No bias adjustments are needed and wiring polarity is
unimportant. Much higher common mode voltage levels can be achieved
with AC coupling because the transformer coupling has a breakdown
rating of 1000 VDC.
However, AC-coupling also has disadvantages. AC-coupled segments
are shorter (700 feet max) and are limited to 13 nodes compared to 17 for
DC-coupling. Also, AC-coupled transceivers operate only at 1.25, 2.5,
5.0 and 10 Mbps, whereas DC-coupled transceivers function over all
standard data rates.
Cabling rules for USB-4000 are similar to those for USB-485. Wire nodes
in a daisy-chain fashion. Refer to
Figure 1
for connector pin
assignments. Termination should only be applied to a device located at
the two ends of the segment. Do not mix AC-coupled and DC-coupled
devices on the same segment; however, bridging the two technologies is
possible with active hubs that have appropriate transceivers.