Control Data Corporation 6000 Series Reference Manual page 27

The computer must know the length of the data block transferred from the remote
station. For example, assume that the data block consists of 100 words; the computer
will then input exactly 100 words. If the computer does more inputs, the last word
(the code word) and the "idle pattern" that follow are accepted as data.
The code word always follows the last word in the data block. When the 12th bit of
the code word is loaded into the A/D register, the last data word has been transferred
to the computer. The 12th bit of the next word ("idle pattern" transmission) sets the
cyclic-error status bit if the code word in the I/ 0 register does not compare with the
output of the cyclic encoder/ decoder. Setting the cyclic-error status bit also sets the
status-all error bit.
To terminate the receive operation after the last word is accepted by the computer,
the data channel must be deactivated or the DSC deselected. The DSC is deselected
by one of the following:
1) Selecting another DSC
2) Requesting status-all
3) Requesting status of the DSC
PROGRAMMING CONSIDERATIONS
Interrupt
An interrupt word from a remote computer indicates that the remote station requires
attention. The local computer checks the 6675 interrupt status bit and selects the
associated DSC to receive
(if
so programmed).
Error Detection
The DSC automatically checks for errors when receiving data blocks. A circuit in the
DSC, the Cyclic Encoder/Decoder circuit, signals the computer if an error is detected.
Both the transmitting and the receiving DSC have a Cyclic Encoder/Decoder circuit.
This circuit generates a code word which is transmitted after the last word of data
block. The receiving DSC compares the code word received from the remote station
with the code word generated at the receiving DSC; if they are not identical, the data
block is in error.
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Rev. A