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The common method of termination is parallel line termination, whereby a resistor is placed
across the differential lines at the most extreme end of the bus (SKB n+1). The resistor value
must be equal to the characteristic impedance of the cable. In practice, the resistor value needs
to be slightly higher than characteristic impedance. A common mistake is to connect a
terminating resistor at each node in the RS485 bus. This action causes line problems when more
than four nodes are connected because the active bus driver is loaded excessively.
The maximum cable length specified for RS485 buses is 1219 meters (4000 feet). Generally, the
longer the cable length, the slower the data transmission rate. All applicable data baud rates used
by the SKB switch operate over the maximum cable length when proper termination is used and
the number of nodes does not exceed 31.
Pin Assignment
The signals for power and communication are connected via the nine-pin D-subminiature serial
connector (Figure 8 and Table 4).
Table 4: Pin Assignment
Pin
Signal
Pin 1
RXTX+
Pin 2
shield
Pin 3
RXTX-
Pin 4
trigger
Pin 5
Attention
Figure 8: Pin Assignment
Description
transmit/receive data positive
side (B)
signal shield
transmit/receive data
negative side (A)
active low trigger signal,
transistor-to-transistor logic
(TTL), multi-dropped, pulled
UP at slave with 100 K
active low attention signal,
open collector, pulled UP at
master with 470 ohm
Functionality
TXA. Differential input/output signal from
the RS485 host.
This line is connected to chassis ground
at the host end and is isolated at the
slave.
TXB. Complement of the differential
input/output from the RS485 interface.
This line is driven by the host processor
(master) to indicate to the slaves that a
trigger-based action is to be executed on
the falling edge of this signal. This action
is usually preconditioned by a request to
the applicable slaves.
This line is driven by the SKB module
(slave) to indicate to the master that it
requires the host attention. This signal is
an open collector signal that is
terminated at the host end by a 470 ohm
resistor connected to V+.
Operating and Maintenance Instructions – 16
(table continued)
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