December 2007
Page 35
90-005-0003 Rev X01
50 Watt Ka Feedmount BUC User Manual
SAbus Protocol
The SAbus interface is a multi-drop, balanced line, asynchronous, full-duplex
communications link designed to interconnect equipment for remote control and
switching applications. Products that are SAbus compatible can be linked together
over a parallel-connected 4-wire circuit without regard to their particular function.
Each SAbus configuration can have one master and up to 63 slave devices (see
Figures 2 and 3). Each slave device is internally configured to respond to a unique
address. A master could be a protection switch, earth station controller, or any
microcomputer or minicomputer that is electrically and operationally compatible with
the SAbus. Since the electrical specifications are very similar to EIA standards RS-
422/485 and RS-449, virtually any computer that meets these standards is capable
of controlling remote devices over the SAbus.
Data Format
SAbus data format supports industry's standard asynchronous ASCII format with
one start bit, eight data bits (7-bit ASCII with the 8th bit set to even parity), and
one stop bit. The ASCII control character subset 00-1F (hex) are used for address,
command, and data characters. The standard bus data rate via direct connect (up
to 4000 feet) is 9600 baud; the data rate for devices connected to a master via
modem is 1200 baud.
Message format and protocol over the SAbus is a derivative of IBM's binary
synchronous communications protocol (BISYNC). The master station sends a
command over the bus to all remote stations. The station whose address is
contained in the second byte of the command message carries out the requested
commands and then replies with a response message containing its own address
and status information relating to its present condition. A remote station only sends
a response following a command from the master containing its unique address.
This prevents bus contention caused by more than one remote device
communicating over the SAbus at the same time.
A remote device ignores all commands that contain parity or checksum errors,
protocol errors, a wrong address, or message overrun errors. A remote device
replies with a not-acknowledged (NAK) character if it receives an invalid command
or data.
