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26
Operation and Control Instructions
Single-byte bit order:
Bit
7
6
5
Double-byte bit order:
Bit
15
14
13
Quadruple-byte bit order:
Bit
31
30
29
Bit
15
14
13
Example:
The 16-bit number represented by 0x1234h is split into two bytes as follows: 0x34h (Byte 0) and
0x12h (Byte 1). In the command or response packet, however, these bytes are contiguous and there-
fore appear as 0x3412h.
Operation and Control using the RS485 Serial Interface
The RS485 interface is a two wire differential bus used to communicate serially with the master
device. The SKB RS485 interface operates over a range of 2400 to 4800 baud. The default baud
rate for the switch is 2400 baud asynchronous, self-clocking.
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 using the RS485 Interface
Up to 31 RS485 unit loads can be connected to the differential bus. Therefore up to 30 SKB
switches and one master controller can be used to construct an RS485 network. Exceeding the limit
of 31 devices excessively loads the RS485 drivers and attenuates the differential signal. The conse-
quence is reduced noise immunity of the bus and an increased error rate. Operation of more than 30
SKB's with a single master is not recommended nor guaranteed.
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.
4
3
2
1
0
12
11
10
Byte 0 (low)
28
27
26
25
Byte 0 (low)
12
11
10
Byte 2
9
8
7
6
24
23
22
9
8
7
6
5
4
3
2
Byte 1 (high)
21
20
19
18
Byte 1
5
4
3
2
Byte 3 (high)
1
0
17
16
1
0
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