December 2007
Page 36
90-005-0003 Rev X01
50 Watt Ka Feedmount BUC User Manual
Message Format
Command messages (see Figure 2) begin with start-of-test (STX) byte followed by a
remote address, a command byte, and multiple data bytes. The end-of-text (ETX)
byte is sent following the last data byte and the message is terminated by a
checksum character. Response messages are identical to command messages in
format with the exception of the acknowledge (ACK) or not-acknowledged (NAK)
character at the start of the message instead of STX. Figure 4 illustrates the format
of the command and response message. A command or reply message may have a
variable length up to a maximum of 132 bytes including delimiters and checksum.
Although most currently implemented SAbus devices require no (or very few) data
bytes, the capability for long messages is built into the protocol so that future
applications requiring the transfer of large amounts of data can be accommodated.
SAbus devices should observe the length of all messages, predefined by their
communication protocol and NAK messages longer than permitted.
STX
ADDRESS
COMMAND
D1
D2
Dn
Dn+1
ETX
CHKSUM
COMMAND MESSAGE:
ACK
ADDRESS
COMMAND
D1
D
Dn
Dn+1
ETX
CHKSUM
RESPONSE MESSAGE: COMMAND ACKNOWLEDGED
NAK
ADDRESS
COMMAND
ETX
CHKSUM
RESPONSE MESSAGE: COMMAND NOT ACKNOWLEDGED-UNABLE
TO EXECUTE OR INCORRECT COMMAND
NSG 380A
Figure 2. SAbus Message Format
Message Delimeters
A command message begins with STX (02 hex), the ASCII start-of-text character. A
message-acknowledged reply begins with ACK (06 hex), the ASCII acknowledge
control character; and a message-not-acknowledged reply begins with NAK (15
hex), the ASCII not-acknowledged control character. All messages end with the ETX
(03 hex), the ASCII end-of-text control character, followed by the checksum byte.
Address Character
The device address must be valid ASCII printable character between "1" and "o", or
31 through 6F (hex); thus, 63 SAbus addresses are possible.
