Contemporary Control Systems PCI20U Adapters - Manual (Page 9)

TD030700-0I
G
9
Segments of -485D connected NIMs can be
extended through the use of active hubs.
Select
an AI Series hub or repeater with a -485D
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. Maintain the same cabling
polarity
as the NIMs by using cable connections that do not invert the signals.
-485X
AC-Coupled EIA-485
The AC-coupled EIA-485 transceiver offers advantages over DC-coupled
EIA-485.
No bias adjustments are needed 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)
than the AC-coupled distance (700 feet) 1 .
The AC-coupled transceiver will
operate at 1.25, 2.5, 5.0 and 10 Mbps while the DC-coupled
transceiver will
operate over all seven data rates.
The cabling rules of the -485X are similar to the -485D.
One three-position
screw terminal connector is provided to attach network cabling.
Wire a
maximum of 13 NIMs in a daisy-chain fashion.
On the end NIMs insert a
jumper at E1 on both -485X daughter boards to invoke 120-ohm termination
resistors (
Figure 7 ).
Termination can also be accomplished by installing a
120-ohm, ¼ watt resistor across pins 1 and 2 of the screw terminals at each
end of the bus segment.
Refer to
Table 3 for connector pin assignments.
Termination should not be applied to any of the NIMs located between the
two end NIMs of the segment.
Do not mix -485D and -485X NIMs together
on one segment; however, bridging of the technologies is possible using active
hubs with the appropriate transceivers. To extend -485X segments, use a hub
as discussed under the -485D section.
Make sure the active hub transceivers
are of the -485X type.
Cable inversion is not of any consequence.
1 Distance figures based on 2.5 Mbps operation.
Figure 6 — Screw
Terminal Connector
Numbering Orientation