Contemporary Control Systems PC10420 Adapters - Manual (Page 12)

TD874100-0IH
12
Segments of -485D connected NIMs can be extended through the use of
active hubs. Select a MOD HUB expansion module 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. 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.
-485 DC Coupled EIA-485 (Backplane Mode)
If the software driver you intend to use sets the COM20020 into backplane
mode, you will need the PC10420-485
version. This version does not utilize the
daughter-board approach. Instead you
will find three sets of jumpers labeled
E3, E4 and E5 which replace the three
jumper functionality of E1 found in the
-485D model. Operation is similar to
that of the -485D version but the bias is
distributed among all the nodes. If bias
is required, place jumpers in locations
E3 and E4 according to Table 6. The
jumper configuration of E3 must match
that of E4. If termination is required,
place a jumper at E5 on pins 1 and 2. If termination is not required, simply
move the E5 jumper to pins 2 and 3. Cabling and expansion rules are the
same for the -485 and -485D options.
Note: When all jumpers are left open, minimal bias is provided by a pair of
10K-ohm resistors.
-485X AC Coupled EIA-485
The AC coupled EIA-485 transceiver offers advantages over 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). The AC coupled
transceiver will operate at 1.25, 2.5 and 5.0 Mbps while the DC coupled
transceiver will operate over all six data rates.
Table 6—Backplane Mode,
DC Coupled EIA-485
Option (-485)
Jumper
E3/E4 # of Nodes
1-2, 3-4, 5-6 2-5
3-4, 5-6 6-15
5-6 16-30