Redundancy Controller
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38
Document Number: 90-005-0030_E
Appendix A
– SAbus
This appendix describes the Serial Monitor and Control protocol for the serial remote control
interface. The SAbus provides monitor and control capability and some flexibility not available in
customized systems solutions.
General Protocol Description
The SAbus interface is a multi-drop, balanced line, asynchronous, full-duplex communications link
that interconnects equipment for remote control and switching applications. SAbus compatible
equipment can be linked together over a parallel-connected 4-wire circuit regardless of their
particular function.
The SAbus subset of RS-422/485 allows up to 63 slave devices with one master to be connected in
parallel with a maximum of 4000 feet between any master and group of slaves.
NSG378A
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, typically any computer that meets these standards can
control remote devices over the SAbus.
RS 422/485 interfaces are unipolar, balanced, 5-volt serial interfaces that connect equipment which
must exchange data over considerable distances with high-noise immunity and high speed.
Standard IC drivers and receivers convert RS-422/485 signals to and from TTL logic levels.
Physical Specifications
The SAbus interface uses four data lines, circuit common, and shield.
No hardware handshaking is used in the SAbus protocol.
All SAbus devices can operate in electrical parallel with a single cable connecting all
devices controlled by a master.
