HP HP-97 User Manual

Theory of Operation HP-97
2-T. By sequentially switching on each cathode driver, only one digit at a
time is actually 1lit. This happens too fast though for the eye to detect.
Each cathode driver transistor is sequentially switched on by the strobe
(STR) signal provided by the display ROM (ROM @) and reset by reset
cathode driver (RCD), which is provided by the ACT. (See figure 2-4.)
2-8. The display of the HP-97 requires a large amount of current. Though
the display ROM decodes the display information given on the IS bus by the
ACT to switch on the current anodes, it alone is not capable of handling
the large amount of current. Anode buffer U3 and U4 are used for this
purpose.
2-9. Every cathode driver in the display that is switched on returns to
ground that corresponding cathode line in the keyboard buffer U4 (PIK)
which will decode and store up to seven keycodes. This allows the operator
to press keys very quickly without waiting for the calculator to catch up.
The PIK will also hold each keycode for a short period to negate the effects
of key bounce.
2-10. PERFORMING A FUNCTION
2-11. Before a key is pressed the ACT is continually asking, "Has a key
been pressed?" If the ACT is not tied up in controlling a calculation,
and a key has been pressed, it will service that key code and the display
will return. (The display is blanked out during printing to conserve
power.)
2-12. The ACT services a key code by first requesting the key code
corresponding to the key that was pressed, from the PIK. The PIK returns
that code to the ACT via the DATA line. The ACT finally will put the
address code on the instruction address (IS) bus. This address goes
to the ROM's that will now send back to the ACT the specific instructions
of how to perform that function and at the same time instruct the PIK as
to what function to print. The ACT will then perform that function on the
numbers in the display; the printer will print (when the print mode switch
is set to TRACE) the function name and the result of that operation.
2-13. TIMING
2-14. The ACT circuit produces two signals for timing purposes: SYNC
for the ROM's, PIK, and CRC, and RCD for the PIK and cathode driver. Along
with the connection through the SYNC line, the ACT is connected to the ROM's
and PIK by the IS (instruction address) bus. The IS bus instructs the data
storage IC to store data sent on the data line from the ACT, and to send data
back to the ACT on the same DATA line. Figure 2-5 shows the timing relation-
ship between the SYNC, DATA, and IS pulses.
2-1

HP HP-97 Specifications

General

Brand	HP
Model	HP-97
Category	Calculator
Language	English