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Operating and Maintenance Instructions
Power / Serial Interface
The nine-pin D-subminiature connector is used for power and serial RS485 interface. The
interface provides power, differential serial (RS485), and trigger signals. The SKB switch has the
receiver and driver installed for the RS485 serial interface but not the software for using this
interface.
The RS485 interface is a high-speed differential bus used to communicate serially with the
master device. The SKB RS485 interface operates over a range of 2400 to 4800 baud and
defaults to read mode. The default baud rate for the switch is 2400 baud asynchronous, self-
clocking. The RS485 interface only requires three lines to connect to the differential bus: shield,
RXTX-, and RXTX+. The interface must be connected to provide power to the SKB device.
The RS485 communication settings are:
•
Eight data bits
•
One stop bit
•
No parity bits
•
2400 (default) and 4800 baud rate
Simultaneous operation of serial and parallel interface is not supported.
Connecting Multiple Switches
Up to 32 RS485 unit loads can be connected to the differential bus, so up to 30 SKB switches
and one master controller can be used to construct an RS485 network. Exceeding the limit of 32
devices excessively loads the RS485 drivers and attenuates the differential signal. The
consequence is reduced noise immunity of the bus and an increased error rate.
The most appropriate method of connecting RS485 nodes is by multidropping the connection
from master to node 1, to node 2, and onward to node n. The bus must form a single continuous
path. Do no put individual nodes at the end of long branches or spokes from the middle of the
bus. If branches are used, make them short. The preferred bus architecture is shown in Figure 7.
RS485 buses usually require line termination when fast transmissions, high data rates, or long
cables lengths are employed. Although no termination is required if the bus cable length is short
and transmission speed is low, it is always recommended.
Terminate bus lines at both ends of the main cable to prevent signal reflection, for example. In
Figure 7, termination is applied at the Master node and SKB n node.
Figure 7: Preferred RS485 Bus Architecture
Operating and Maintenance Instructions – 15
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