Sierra Video VS User Manual download pdf (Page 38)

SIERRA VIDEO SYSTEMS

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.

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.

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Sierra Video VS User Manual Page 38