Sierra Video Tahoe Series 16 - Manual (Page 28)

Sierra Video Systems
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The protocol uses only 7-bit ASCII characters. The 8th bit of received characters is
treated as if it is 0. Within the command string, certain ASCII characters may be present
and are ignored: any ASCII character whose code is less than the SPACE character
(includes all control characters and the SPACE character) and the DEL (ASCII 7F)
character. Alphabetic characters within the command string may be in either upper-case
or lower-case letters. The router always sends upper case characters, except for
character strings such as input, output, and level names, which may have lower case
characters in them.
When sending commands to the router, SPACE characters are optional, but if used may
only appear before and after each individual command and NOT embedded within an
individual command. Within command strings sent from the router, a single SPACE
character appears before and after each individual command. SPACES may also appear
in character strings, such as input, output, and level names.
Certain commands (R, Q, L, and G) have character strings that appear as arguments.
The first three, “R”, “Q”, and “L”, have character strings only in commands sent from the
router, and these character strings are always terminated with a ~ (tilde) character. No
special character marks the start of these strings, they simply begin at the appropriate
point within the command. The “G” command, on the other hand, uses the ~ (tilde)
character to mark both the start and end of a character string argument.
Just before the router begins executing a command string, it sends a leader (**) to the
host (the same number as are required in commands from the host). As it executes the
commands, some of them may generate additional output back to the host. These
command response characters are always preceded and followed by a space character,
making the response string easily human-readable.
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 the trailer (!!)
and a CR (carriage return, ASCII 0D) 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 router returns
the string " ERROR ", followed by an optional descriptive string followed by a string of
trailer characters and a CR character, to the host. An error can be caused by an unknown
command name or bad arguments to a command.
The simplest possible command string would be:**!! which consists of the leader and
trailer characters but no commands between them. This command string would generate
the response:
** OK !!
This can be useful for verifying that the serial link to the router is operational. In routers
requiring only one leader/trailer character, the simplest command string would be: *!
which would generate the response:
* OK !
(To determine whether a particular router uses one or two leader/trailer characters, send
it "!!**" and check the response to see which of the above two responses it is. It won’t hurt
to always use two even if only one is required.)
The simplest error response is one with no optional descriptive string. For example, this
command string:
** XXX !! might generate this response from the router:
** ERROR Syntax: No Number:XX !!
The descriptive string always ends with a colon and up to three characters from the
command string that caused the error. Generally, the error can be assumed to have
occurred just before these characters