DEC DECtape 551 Manual page 34

RWB, causes the exclusive OR of the LB and the RWB to replace the contents of the LB. One
time pulse later, on TP3, the RWB contents are cleared in preparation for the next 6-bit byte.
When the checksum is about to be written at the end of the current data block, the 1 which is
shifting through the TDATA register shifts into TDATA6. Once again the NOT TDATA 1 or 6
level inhibits the request for another 6-bit byte from the data control. On the TP4 that would
have requested a 6-byte, the RWB
~(-- LB(l) pulse is generated by the logic at coordinates
A3 of drawing RWBC. Thus the first 6-bit byte to be written out in the checksum slot is the
contents of the LB. As with any other 6-bit byte, the contents of the RWB ,gre then exc lusive "
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ORed with the contents of the LB. Because any number exclusive bRed with itself yields......"
the LB becomes cleared. Therefore, the DECtape control fills out the checksum slot with ......,
although the contents of the LB are transferred twice more to the RWB.
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Skipping Over the Interblock Zone
If more than one data block is to be written at one time, the DECtape control must skip over
the interblock zones so as not to disturb the reverse and forward block marks written there. If
more data is ready in the data control, the NAND gate in drawing RWBC near coordinates B4
receives the following signals: DC SELECT 1 signifying that device 1 (the DECtape control) is
slected by the data control; NOT DC DA RQ signifying that a 36-bit word is waiting in the
data accumulator for transmission to the DECtape control; UT DATA signifying f'hat the DECtape
command is to write data blocks; and RW ACTIVE signifying that data transmission is in progress.
The AND of these four signals is RW DATA CaNT. In order to suspend data transmission for
the duration of the interblock zone, the tristable flop is set to the RW RQ state by the output
of a capacitor-diode gate enabled by RW DATA WR CaNT and pulsed by the transition of
TDAT A6(O) going to 0 at the end of the checksum slot: Data transm ission resumes in exactly
the same way that transmission was begun initially; a data-sync mark-track code is read in the
next dat'a block, thus supplying another 1 to shift through the TDATA register.
End of Data Transmission Operations
Once the DECtape control begins to write a data block, it must write the entire block in order
to have a val id checksum at both ends of the block. Writing ceases when the data control 'is
no longer supplying data to the DECtape control. This condition is detected as shown on
3-10