DEC DECtape 551 Manual page 26

both the UT CONO SET pulse (O.4-microsecond width) and the UT CONO SET LONG pulse
(l.O-microsecond width). All circuits which receive UT CONO SET LONG trigger on the
trail ing edge, thereby achieving a 1-microsecond delay after UT CONO SET.
The command register consists of the Type 4217 Flip-Flop modules shown on drawing UTC1. The
UT CONO CLEAR pulse clears the entire register simultaneously, and the UT CONO SET pulse
reads the I/O bus data I ines into the register.
Initial Time Delay
The initi'al time delay circuits are shown at the bottom of drawing UTC1. The outputs of the
two flip-flops that store the delay-specification bits, UT TIMEO and UT TIME1, are applied to
a NOR gate at 1 B5 to produce the UT TIME level whenever a
no~zero
deloy time is required.
The outputs of UT TIMEO and 1 a Iso enable one of the three delay control units at 1 B3. UT
START is a controlled delay, triggered by the trailing edge of UT CONO SET LONG. The
output of UT START remains in the 1 state from UT CONO SET LONG time to the end of the
requested delay period. The transitions of the UT START delay provide delayed pulses required
in portions of the circuits discussed later.
Tape Unit Selection
Four flip-flops of the command register control which DECtape drive is selected. The outputs
of these four fl ip-flops, UT UN ITS SELECT and UT UN ITS 0 through 2, are appl ied to a 4-bit
binary-to-BCD decoder. Only the 0 through 7 decoder outputs are used. The extra fl ip-flop,
UT UNITS SELECT, makes possible the deselection of all units. Because the BCD decoder in-
terprets correctly only 10 of'the possible 16 input combinations, having UT UN ITS SE LECT con-
tain 0 is not sufficient to deselect all units; UT UNITS 0 and 1 must also contain O. Ground is
applied to the one line in eight corresponding to the selected unit (or, if no unit is selected,
all lines are at -3 volts). The DECtape transport that is dialed to the grounded line responds
to the transport control signals and connects its read-write heads to the head signal lines.
The normal ground input to pin D of the Type 4671 is disconnected and a 2-ohm resistor to ground
is inserted instead. This resistor develops an analog signal, UT UNIT CHECK, that is measured
3-2