SIERRA VIDEO SYSTEMS
27
Communication
Protocol
Introduction
The protocol uses the 7-bit ASCII character set, usually sent over an RS232-C or RS422 serial
link — 9600 bps is recommended with 8 data bits and no parity. The protocol is compact, with few
characters required to cause switch changes to occur. It is also human-readable and thus easy to
understand and use. Several different crosspoint switch request commands are defined, so that
the one that is most compact for any given switcher and application can be chosen.
The protocol is useful with both very small and very large routing switchers. The sizes of the
numbers representing inputs, outputs, and levels are not fixed, but can be as large or small as
necessary. Special provisions allow numbers to be packed one after another with no intervening
delimiter character, in order to make the protocol compact, as long as each number is the largest
size necessary for that particular switcher.
All input, output, and level numbers begin at number 1, not 0.
Generic Protocol
Commands are sent to a routing switcher in a group called a command string. A command string
can contain zero or more commands, limited only by the size of the receive buffer of the router,
whose size depends on the particular router model.
A command string consists of a leader string of asterisk characters, zero, or more commands,
and a trailer string of exclamation marks. Larger routers require two leader (**) and trailer (!!)
characters, while small routers require only one, in order to make the protocol compact for those
routers. The remainder of this document gives examples using doubled characters. Note that two
leader/trailer characters may be sent to small routers even when only one is required, and they
will still work fine.
If a leader character (**) is encountered within the command string being processed by a router,
the command string up to that point is discarded and a new command string is expected. This
Chapter
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