DEC DECtape 551 Manual page 40

Active Error Check
If the DECtape control is active when the tape enters the interblock zone, an error has occurred.
For example, if the DECtape control is writing data blocks, the active mode is in effect while
writing is in progress. After writing the checksum, the DECtape control must enter the request
state to avoid writing over the block marks written in the interblock zone. Similarly, if the
DEC tape control is active when the tape leaves the interblock
~one
and enters the data zone,
an error has occurred. Both errors are detected by the circuit near the center of drawing UTE.
Except in the read or write all modesi'n which it is desired to read or write through the inter-
block zone, the UTE ACTIVE ERROR level is asserted if the error condition arises. The pres:nce
of this level has two effects. First, on TP2 the UT INFO ERROR fl ip-flop on drawing UTC2 is
set to 1 to indicate an error condition to the arithmetic processor. Second, the tristable flop
on drawing RWBC is set to the RW NULL
s~ate,
also on TP2, to terminate data transmission and
prevent the pO'ssible loss of data.
Illega I Operation Error Check
The illegal operation error occurs when the DECtape control receives some command that can-
not be accompl ished due to the settings of the operator's control switches. For example, if a
write operation is selected and the transport is in the WRITE LOCK mode, the UT WRITE PRE-
VENT level is present. When the RWB
(J)~
WB pulse occurs, indicating that a write oper-
ation is being attempted, the UT ILLEGAL OP flip-flop is set. As previously described, writing
is inhibited by the UT WREN DATA level.
Two other error stiuations exist: two or more units (or zero units) are dialed to
~he
selected
transport number, or the UT BTM SWITCH position does not agree with the DECtapecommand.
The analog circuit that detects when the wrong number of units are dialed to the selected unit
number is described under Tape Unit Selection. Two NAND gates at the bottom of drawing
UTE detect when the position of the UT BTM SWITCH disagrees with the
DECt~pe
command.
The two error conditions are ORed to form the UTE SW ERROR level. This level is tested at the
completion of the initial delay and sets the UT ILLEGAL OP flip-flop (see drawing UTC2) if an
error is present.
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