Sierra Video 503108 RS-232 - Manual (Page 4)

4
Sierra Video Systems • P.O. Box 2462 • Grass Valley, CA 95945 • www.sierravideo.com
503108 RS-232 INTERFACE
The terminal protocol echoes each digit character
that is entered, so the terminal should be set so that it
does NOT echo characters as they are pressed.
HOST COMPUTER PROTOCOL
The protocol is designed to use the 7 bit ASCII
character set (00-7F hex), usually sent over a
RS-232C or RS-422 serial link. The high- order
data bit is ignored on all received characters, and
is 0 on all transmitted characters.
The protocol is designed for use over a reliable
channel. Noisy channels such as modem connec-
tions over the telephone network should use an
error-checking and/or error-correcting protocol,
including such things as packet checksums or
CRC’s. Newer modems include such communica-
tion schemes as part of their normal operation,
and are recommended for these applications.
The protocol is designed to be compact, with few
characters required to cause switch changes to
occur. It is also designed to be human-readable
and thus easy to understand and use. Finally, it is
a modified superset of the existing SVS host
protocol, allowing a degree of compatibility with
those existing systems.
Several different 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 designed to be 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.
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 switcher, which should be large enough to
hold a command string for setting the state of the
entire switcher matrix.
A command string consists of a leader, zero or more
commands, and a trailer. If a leader character is
encountered within the command string, the
command string up to that point is discarded and a
new command string is started. Once a complete
command string, up to the trailer character, is
received, the routing switcher executes the com-
mands within it.
Within the command string, certain ASCII charac-
ters may be present and are ignored: any ASCII
character whose code is less than or equal to the
SPACE character, and any whose code is greater
than the “~” (tilde) character. Alphabetic characters
within the command string may be in either
uppercase or lowercase letters.
Just before the switcher begins executing a com-
mand string, it sends a leader character to the host.
As it executes the commands, some of them may
generate additional output back to the host. After
the command string has been executed, the routing
switcher returns the string “OK” (with a single
space character before and after the word “OK”),
followed by a trailer character (~) and a CR
character, to the host This indicates that the
command has executed successfully. If an error
occurs within any command of a command string,
the remainder of the command string is ignored
and the switcher returns the string “ERROR”,
followed by a trailer character and CR character,
to the host. An error consists of an unknown
command name or bad arguments to a command.