RCA 301 Programmer's Reference Manual page 29

If an attempt is made to select a card from a magazine position that does not contain a magazine, an Illegal
Operation indication is given. The fact that a magazine is not present will be sensed by a switch located in each
magazine position.
Automatic Removal of Unserviced Cards
Whenever a card remains unserviced for over 32 revolutions of the capstan, the card will be automatically
removed, returned to its magazine.
Read and Write Checking
A Read-after-Write parity check and missing bit check is automatically made as each block is written. Failure
of this check will cause termination of the Write, and an indication is given.
While reading, each character read will be checked for proper parity. Any single bit error in a specific character
will be automatically corrected before being transmitted to HSM. If a parity error occurs that cannot be automatically
corrected, an (57) will be transferred to HSM, a parity error indication will be set, and the remainder of the
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information in the block will be transferred but may not be valid. If more than one block is being read, information
transfer will resume at the beginning of the next block.
Address Data Checking
Address data is parity checked. When bad parity is detected, the unit automatically become inoperable.
Preamble Checking
A synchronizing preamble is recorded in front of each block written. At the beginning of a Read and Read-
after-Write, this preamble is used to synchronize the operation. If synchronization is not accomplished, a preamble
error is indicated.
Interlocks
As soon as the first card is fully extracted, a preselect of a second card can be executed. The second card will not
be extracted until the first card leaves the capstan. If the card returning is in the raceway (in position to be reloaded
into the magazine), the reload will be delayed until the extract is completed. If the reload has been started, the
extraction will be delayed until the reload is completed.
If a card is in the return cycle, and there is a card on the capstan and a preselect is attempted, the preselect
command will be accepted but not executed until the reload of the card in the return cycle is completed.
If there is a card in the return cycle that has not reached the reload point, and no card is on the capstan, a card
may be selected and extracted immediatelv, while the card on the return path is delayed in its reload operation.
At no time will there be more than two cards in motion.
Card Extract Counter
The Card Extract Counter is mechanized by recording a 15-bit binary number in a special track on the card.
During normal operations, the count is read, decremented by one, and rewritten. A check is made to indicate when
the count reaches zero. When the count reaches zero, a special (equal) bit is recorded. On the next extraction, the
system recognizes the equal bit and records an overflow bit. On all subsequent extractions, overflow is reported and
no writing takes place.
These indications can be sensed by use of the Standard 301/3301 Sense instruction as follows:
Option 2 — = 1 Test for count equals limit.
2 '-
2 3 = 1 Test for count has exceeded limit. (Set each time card is placed on capstan after the count
has been exceeded.)
Five (5) toggle switches are used to set up the amount to be prewritten on the card. These switches are located
on the rear of the maintenance control panel.
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