DEC DECtape 551 Manual page 31

The read write buffer (RWB) appears in drawing RWB. One of two sets of capacitor-diode gates
reads the 6-bit
byte
from the appropriate end of the data accumulator into RWB. Simultaneously,
the data control starts to shift the next 6-bit byte into position. Information from the left end
6
of the data accumulator enters via a plug at 1 D25; the right end enters via 1 D24.
The sequence of clearing RWB on TP3 and refill ing it on TP4 whenever TCT contains 0 continues
unti I the 36-bit block mark is written. Since TCT complements on every TP2, a new 6-bit byte
is requested every two bit times.
Data Write Control
At the next TPO, the data must enter the write amplifiers and writing must commence. If tape
motion is in reverse, the complement of RWB must be written (the obverse is formed previously).
Whenever the DEC tape control is writing, the circuit in the upper left corner of drawing RWBC
generates a RWB(J) )
we
pulse every TPO. This pulse, in turn, triggers either the
WB .( RWB(J)FWD or WB ( RWB(J)REV pulse depending on the direction of tape
motion.
The write buffer (WB) appears in drawing RWB at the upper right. Which of the two 3-bit bytes
is currently to be written is determined by the state of TCT. Six NAN D gates at 1 A 18 select
the proper byte and present the resulting data to the write inputs of the Type 4523 Manchester
Reader and Writer modules. The gates on these modules/jam the 3-bit byte or its complement
into the write buffer fl ip-flops as determined by the WB ( RWB(J)FWD or WB
<
RWB
(J) REV pulses.
One other signal required before write current is applied to the read-write heads; that signal is
UT WREN DATA. This level which is asserted so long as data continues to be written, is inter-
locked with the WRITE LOCK switch on the selected transport. UT WREN DATA is developed
on drawing UTC2. The UT WREN flip-flop at the top center of the drawing synchronizes UT
WREN DATA with the necessary time pulses. This flip-flop is sei-
t..J
1 on the first TPO after
the DECtape control becomes active and is not set back to 0 unti I TP4 after data transm ission
ceases. The WRITE LOCK switch of the selected transport is connected across pins Band J of
plug 1 B22. If the switch is open, writing is prevented and the UT WRITE PREVENT level is
3-7