Contemporary Control Systems AI-USB Adapters - Manual (Page 10)

TD030500-0IB
10
Ground
For EIA-485 DC operation, it is important that all devices on
the segment be referenced to the same ground potential so the
common mode voltage requirement (+/–7 Vdc) of the EIA-485
specification is achieved. This can be accomplished by a
separate ground wire tied to all devices or using the third wire
ground of the power connector. Alternately, connect the DC
common of each node to a cold water pipe. Connected systems
having different grounds, can cause unreliable communications
or can damage the drivers. Therefore, adequate grounding
must be implemented. A ground connection can be made to
the middle terminal of .
485X 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 the 17 nodes of DC-coupling. Also, AC-coupled
transceivers operate only at 1.25, 2.5 and 5.0 Mbps while DC-
coupled transceivers function over all six standard data rates.
.
Cabling rules for 485X are similar to those for 485. Wire nodes
in a daisy-chain fashion. On the end devices, insert a jumper at
E1 on both 485X daughter boards (
Figure 4 ) to invoke 120
ohms of termination or leave the jumpers open and install a
120-ohm, ¼ watt resistor across pins A and B of the screw
terminals at each end of the bus segment. Refer to for
connector wiring. Termination should not be applied to any
device located between the two ends of the segment. Do not
mix 485 and 485X devices on the same segment; however,
bridging the technologies is possible with active hubs having
appropriate transceivers. To extend 485X segments, use a hub
as discussed under the 485 section, but the active hub ports
must now be 485X types. Cable inversion is insignificant. Be
sure that the data rate is the same for all network devices (see
Figure 5 and its associated explanation).