Redundancy Controller
Page
41
Document Number: 90-005-0030_E
1
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NO STX
EXECUTE C
OMMAND
POWER ON
INC
OR
REC
T C
HEC
KSU
M
CORRECT
CHECKSUM
ET
X
STX
VALID
ADDRE
SS
STX
IN
V
A
L
ID
A
D
D
R
E
S
S
S
T
X
IN
V
A
LID
C
H
A
R
A
C
T
E
R
IN
C
O
R
R
E
C
T
M
E
S
S
A
G
E
LE
N
G
T
H
VALID
CHARACTERS
NSG 381A
At least a 10-bit time delay must be inserted between command messages in order to wake up a
remote device. Once the device is awakened by data on the bus, it looks for STX followed by its
address. If it does not see its own address, it ignores the rest of the message by going to sleep and
remains in that state until the serial data line idles for at least 10-bit times or approximately
10 milliseconds.
All SAbus compatible devices must respond to a command 0 (30 hex), with 6 data bytes of ASCII
characters in the following form:
ACK ADDR 30
D1
D2
D3
D4
D5
D6
ETX
CHSUM
where D1 through D4 are four ASCII characters representing the model number and D5 and
D6 are two ASCII characters representing a software version number.
If more than one command is required to obtain status information of device's functions that can
cause a change bit to be set. Then the device must implement a clear change bit command and this
must be the only command which causes the change bit to clear. If several commands have to be
executed in order to set all the information that can cause a change bit to be set, then multiple
change bits may be used to reduce the bus traffic.
Wherever possible, SAbus numeric data should be sent encoded as ASCII data characters and only
in cases where it cannot be avoided, numeric data should be sent in binary or BCD packed format.
Status bits in data bytes (i.e., change bits, alarm bits, etc.) should occupy no more than four bits in
the low-order nibble. The high-order nibble should be set to 03 to guarantee that the byte will
contain a printable ASCII character.
The slave state diagram presents the required protocol implementation at the slave device that
guarantees the proper transfer and processing of communication messages sent by a
master/controller over SAbus.