GENERIC PROTOCOL
47
indicating that output 21 is NOT locked. Or, the response might be:
** B21,6741,1 OK !!
indicating that output 21 is locked with password 6741. To clear this lockout:
** B21,6741,0 !!
To lock output 96 using password 439:
** B96,439,1 !!
If successful, the response would be:
** B96,439,1 OK !!
If output 122 becomes locked using password 235, the following change report would be sent by
the router:
** B122,235,1 !!
“F”: Field Delay
The command "
F
" is used to specify the delay between the time a crosspoint change request is
received by the router and the time the crosspoint switch actually occurs. It must be followed by a
number giving the number of video fields of delay desired. If the number is smaller than the
smallest delay that the router can handle, the smallest delay is used instead. If it is larger than the
largest delay the router can handle, the largest delay is used instead. Note that this command
does not cause a delay in command processing, as the “
D
” command does.
To understand this command more fully, consider the way that router software will typically
handle a crosspoint command. The last character of the command string, the final “!”
(exclamation) character, is received somewhere in a particular video field, call it video field 7. The
router parses the command string and, for each crosspoint it contains, it puts the crosspoint in a
buffer that is marked to be delivered to the crosspoint hardware on a particular video field.
Suppose that previously, an “F5” command has been received. Then crosspoint commands
whose final “!” command string character was received on video field 7 would be placed in a
buffer that is marked to be delivered to the crosspoint hardware at video field 13 (7+5+1=13).
To understand the reason for adding 1 in the previous sum, consider an “F0” command: it would
ask for output at the very next video field, field 8 in our case. So, it is necessary to add the “F”
argument plus 1 to the field number on which the crosspoint command is received to get the field
number at which the crosspoint will be output.
A typical router will have a minimum delay that is between 1 and 2 fields. Suppose a crosspoint
command is received just before a vertical field mark. The software may be able to prepare the
crosspoint data and send it to the hardware when that vertical field mark occurs, but the hardware
itself typically has a one-field delay in it, so the soonest that such a crosspoint would switch would
be one field (plus a little) from when it was received. If the command were received towards the
beginning of a field rather than the end of a field, the delay would be closer to two fields. Industry
parlance is to call this a one-field delay, because only
full fields
of delay are counted.
