RCA 301 Programmer's Reference Manual page 184

The 3^88 unit has two levels of storage for card select information, so that a card preselect function can be
performed. The card preselected will not be extracted on to the raceway until the trailing edge of the returning card
passes the detection station approximately 10ms ahead of the read-write heads.
Card and block selects should only be issued when the device will accept them. If the device can not accept the
illegal operation
select, an occurs. This causes the termination of the Select instruction. A Card Select (or Preselect)
is permissible only when the specific unit indicates that the select is permissible. A Block Select (or Preselect) is
permissible only when the specific unit indicates that a Block Select is permissible.
A Block Select with no card on the capstan will set the illegal operation bit. The 32
unserviced revolutions
remains set if a 32
Whenever one of the 3488 units completes its card or block selection, the 8-10ms
of the device will be reset.
When a Block Preselect is accepted, Block Select is not permissible until the Block Preselect is executed. Any
further block selects to the same unit during this period of time results in an illegal operation. A Block Preselect should
not be-attempted unless a ca rd is on the capstan. A card leaving the capstan for any reason will cause a
permissible
condition.
Final Register Contents
( A ) = ( A ) ,
t
( B ) t = ( B ) i
XV-4
condition is present.
unserviced revolutions bit
select complete
indication
block select not