Sierra Video Tahoe Series 16 - Manual (Page 27)

Sierra Video Systems
21
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 ensures that a router will always act on a complete command string sent
to it, even if the previous one was never completely received.
When a command string is received, it is not acted upon (but rather, is merely buffered
up) until the final trailer character (!) character of the command string is received. At that
time, the routing switcher begins to execute the commands within the string.
Chapter 4