Sierra Video Tahoe Series 16 - Manual (Page 40)

Sierra Video Systems
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however, will add one more field to the minimum possible delay. Thus, delayfields
specifies the number of full fields of delay between end-of-crosspoint-command-string-
received and crosspoint-switch-occurs. Note that the actual minimum value of delayfields
depends on the particular router model.
Routers typically have a limit to the number of crosspoint commands they can process in
one field. First, there is an inherent delay in sending the command to the router, but
beyond that, the router requires time to parse the command and buffer up the crosspoint
data, plus it requires time to deliver the buffered data to the hardware when the desired
video field arrives. Each individual router has documentation to describe its limitations on
how many crosspoints it can process in a given amount of time.
Larger values for delayfields give the router more time to process commands. Although
the long-term average number of crosspoints that can be processed per unit of time is
unchanged, a larger delayfields value can improve router performance during a short
burst of many crosspoint commands. For example, suppose a large number of crosspoint
commands is sent to the router in a single large command. If delayfields is small, the
router typically wonít have time to parse and process all these crosspoint commands and
place the data in the crosspoint delivery buffer before the target video field arrives. By
making delayfields larger, the user can give the router more time to process the
crosspoint commands.
If too many crosspoint commands are received and the router is not able to process them
fast enough, it will output the crosspoint connections as soon as it can. Unexpected
delays in crosspoint output are a sign that the router is being pushed beyond its limits.
The fielddelay value applies to the entire router, not just to the control port on which the
“F” command is received. It is therefore recommended that a single value be settled on
for the fielddelay value, rather than changing the value constantly depending on needs.
Once changed, the router records the value in non-volatile memory and uses it each time
it is powered up, so it is only necessary to change it one time.
Even though a crosspoint isnít changed until the fielddelay time has elapsed, the router
records the new crosspoint state immediately upon receiving the crosspoint change
request,
so a controlling device may receive a report of a crosspoint change before the
change has actually taken effect, and this is more likely to happen the larger fielddelay is.
Since routers currently make no guarantees about when they will report a crosspoint
change anyway, this behavior is usually of no concern. There is a case where this could
cause problems. If the fielddelay value were to be changed while two different devices
were changing the same output, it is possible for the router to report the incorrect input
value for that output. This would happen if the earlier device that changed the output did
so before the fielddelay value was changed, and the later device that changed the output
did so after the fielddelay value was reduced but soon enough that its input value would
be sent to the crosspoint hardware before that of the earlier device. A bit later, the earlier
deviceís input value is sent to the crosspoint hardware, but the router has recorded the
later deviceís input value as being the one in effect. To prevent this scenario, we
recommend that an appropriate fielddelay value be chosen, set, and left alone.
Here is an example of an “F” command:
** F5 Y1,5 X2,6,3 !!
This says that input 5 is to be connected to output 1 on all levels and input 6 is to be
connected to output 2 on level 3, after a delay of 5 fields from the beginning of the field
that follows receipt of the “!” character.