Grass Valley Performer SD - Manual (Page 175)

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Serial Protocol Descriptions
A single I/F ITEM RESPONSE message is sent containing all
values (wrapped in BEGIN/END) requested via READ or
UPDATE messages since the last such response message was sent.
Two separate READ messages back to back will NOT produce two
I/F ITEM RESPONSE messages containing the separate data
items, but rather will produce a single message containing them.
The READ message will result in all the values being read
simultaneously, just as with the SRDR (3F/11, Simultaneous Read
Response) message.
The destinations-to-source and sources-to-destination
information fields are sensitive to whether or not the level, source,
and/or destination pointers are set to the wildcard values (FF,
FFFF). If so, reading these information fields results in multiple
I/F response values for these I/Fs; one for each wildcard
replacement combination. The SMPTE spec is a bit unclear about
how the wildcarding should apply, but this seems to be the correct
way.
Note that it is not possible to read the value of a single crosspoint,
only of entire rows or columns of the crosspoint matrix (using
sources-to-destination or destinations-to-source). Setting the
matrix, level, and destination pointers, and then reading the
source pointer, will NOT give the source connected at that matrix/
level/destination. Instead, it will give the current value of the
source pointer.