December 2007
Page 37
90-005-0003 Rev X01
50 Watt Ka Feedmount BUC User Manual
Command Character
The command (CMD) character immediately follows the device address and specifies
one of a possible 80 different commands for a particular device. Values from 30 to
7F (hex) are allowed. Commands may be completely device dependent with the
exception of command 30 (hex), which must cause a device to return its six-
character device type and command 31 (hex) which is a status poll.
Command and Reply Data
A command or device reply may contain from 0 to 128 data characters and is
restricted only to printable ASCII characters 20 through 7F (hex).
Check Character
The last character of any SAbus message is the check (CHK) character. This
character is simply the bit-by-bit exclusive OR of all characters in the message
starting with the STX character through the ETX character. This forms a longitudinal
redundancy parity check over the entire message.
Message Timing
Different devices will require varying times to execute commands from a master. A
receiver, for example, may be instructed to change frequency and may require up to
a second for the synthesizer to lock. This should not prevent it from immediately
acknowledging the command. The NAK or ACK reply does not signify that a function
has actually taken place, but only that the message was received and understood.
A status reply should indicate if a device is executing a time-consuming function.
A remote device must begin responding to a command within 100 milliseconds after
receiving the last character of the command and no more than 10 milliseconds must
pass between each character. If the remote device does not respond within this
time, the master/controller should attempt to re-establish communication by re-
polling this device at least once. Figure 3 shows a remote device SAbus state table
and timing requirements.
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.
