Contemporary Control Systems USB222 Adapters - Manual (Page 6)

TD040900-0II
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.
Two models support AC-coupled EIA-485 segments. The USB22-4000
provides dual RJ-45 jacks, whereas the USB22-4000/S3 offers a 3-pin
screw terminal.
Cabling rules are similar to those for DC-coupled NIMs. Wire nodes in a
daisy-chain fashion. Refer toFigure 1 Figures 1 and 2 for connector pin
assignments. Termination should only be applied to devices 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.