HP 13255 Technical Information page 27

13255
Cartridge Tape Module
13255-91032/25
Rev AUG-Ol-76
3.1.2
W~ITE
CURRENT CIRCUIT - READ/WRITE PCA.
3.1.2 •. 1 The write current circuit provides the current sOUrce for either head
as selected
by
the unit function decoder. In addition, the RIP signal
is generated allowing the firmware to determine whether a cartridge is
protected.
3.1.2.2 When RE is low (write) V17, Pin 4 is high and Q15 turns on. This, in
turn, drives Q16, the current source, on. When RE is high (read),
V17, Pin 4 is low and Q15 is cut off which in turn cuts off Q16. The
emitter current of Q16 is primarily determined by the +12 VOlts supply,
R42, CRI3, and is nominally 9.85 mAe When the current source (QI6) is
off, the RIP driver (017) is turned on through R42, CRII, CRI0, R44.
3.1.2,,3
~hen
the current source is on and supplying current to the head, the
emitter of Q16 is at approximately +4.6 volts and CRll isolates the
base of Q17 from this voltaqe. The collector of Q16 is at a low volt-
age when anyone ot U3's outputs (Pins 1, 2, 3, or 4) is low, then CRl2
removes the drive to the base of Q17 cuttinq it oft. If the current
source is on, and not supplying current to the head (i.e., the file
protect switch is open, or the head leads are broken), the collector of
Qlb is then connected to a high resistance path (CR12, R44, R49) and
Q16 saturates causing most of its emitter current to flow out its base
into
015
and CR13 to ground. Then the collector voltage ot
Q16
which
is now about the same as the emitter voltage, drives 017 on through
CR12 and R44. The two paths driving Q17 assure that RIP is true only
when head current Is actually drawn.
3.1.2.4 when recording data, however, RIP is not at a steady dc level. The
head, being inductive, opposes an instantaneous current change. For a
brief instant, at each current reversal, the -L(di/dt) is such that Q16
saturates and stays saturated until the rising current through the head
reacnes a sufficient value to allow Q16 to come out of saturat10n.
Since, as explained above, Q17 is triggered on and then Q16 Is satu-
rated, narrow negative going pulses, approximately 10 microseconds
wide, appear at the RIP output. RIP is at a steady high level only
when writing gaps.
3.1.3 READ SELECT SWITCH - READ/WRITE peA.
3.1.3.1 The read select switch (U16) Is a FET switch used to connect the head
on the selected CTU Transport Assembly with the preamp in the read
amplifier circuit when the Read/write peA is in the Read mode.