Data protection with “Block Check Character BCC”
“Transfer bllockwise” and simultaneous program execution requires data check procedures (IS0 1 155 and
IS0 2111). For this reason, during the “transfer blockwise” mode of the TNC 151/TNC 155, a parity check
of the complete transmitted block (length parity) is performed in addition to the’parity check of the
individual characters (cross parity). This is performed with the “Block Check Character BCC”, which completes
the individual bits of the transmitted character of a data block for an even length parity.
At the end of a block, the control checks data transmission via the BCC. For this, the TNC 151/TNC 155
generates a IBCC and compares this with the BCC being received. If the generated BCC and the received BCC
are identicali, the control signals ACK to the peripheral unit.
If the BCC’s are not identical, the control signals NAK, and the block must be re-tralnsmitted to the peripheral
unit. This plrocedure is repeated up to 3 times and the following error message is then displayed:
TRANSFERRED DATA INCORRECT
With data output the control can transmit a BCC to the peripheral unit. The TNC 151/TNC 155 then waits
for the signal NAK or ACK from the peripheral unit. If the peripheral unit transmits ACK, the next program
block is transmitted.
If, however, the peripheral unit transmits NAK, the program block is repeated.
The control repeats the same block up to 3 times. If the peripheral unit transmits NAK each time, the
following error message is displayed:
ME: PROGRAM INCOMPLETE
If the calculation of the BCC for “Transfer blockwise” results in a figure which is smaller than HEX 20
(control character), a character “Space” HEX 20 is additionally transmitted before ETB.
With this, the BCC is always greater than HEX 20 and no longer corresponds to a control character.
If the BCC is not checked, this function need not be selected - Machine parameter MP 222.
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