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HP 35 internals

Posted by Tony Duell (UK) on 18 Sept 2003, 9:15 p.m.

There are 2 PCBs, connected by 25 pin contacts. The upper PCB contains 
the display, keyboard, display drivers, etc. The lower PCB contains the 
processor and firmware ROMs. The connections between the PCBs are : 

o-- Disp 0 o-- Disp 1 o-- KC1 o-- Disp 2 o-- Disp 3 o-- Disp 4 o-- Phi_1 o-- Phi_2 o-- KC2 o-- Vbsw o-- Vcc (+8.2V) o-- Gnd o-- Gnd o-- RCD o-- KC3 o-- KR1 o-- KR2 o-- KR3 o-- KC5 o-- KR4 o-- KC4 o-- KR5 o-- KR6 o-- KR7 o-- KR8

KC = Keyboard Column line. KR = Keyboard Row line Disp = display data bus from A&R chip to anode driver RCD = Reset Cathode Driver - start display scan Phi_n = 2 phase system clock Vcc = 8.2V power line from PSU on logic board to cathode driver. Vbsw = swiotched battery voltage.

There are 2 custom chips on the keyboard/display PCB.

1820-0853 (anode driver and system clock)

---------- Disp0 --| U |-- Ag Disp1 --| |-- Ae Disp2 --| |-- Ah Disp3 --| |-- Af Disp4 --| |-- Ad Step --| |-- Gnd Phi_1 --| |-- Ab Phi_2 --| |-- Ac LC --| |-- Aa Rset --| |-- Vbsw ----------

Disp = Display data bus (as above). A = display anode drive lines [1], segments named conventionally (h = decimal point) Phi_n = clock output to logic board Step = step output to cathode driver (display scanning) LC = oscillator components (LC series circuit from here to ground) Rset = resistor from here to ground, appears to be part of low battery circuit

[1] There's an inductor of 130uH (or 68uH for the point) from each anode line to Vbsw. Display runs from back-emhf from these inductors when the anode driver turns off. Neat!

1820-0854 cathode driver (essentially a 15 stage shift register for display scanning).

---------- k9 --| U |-- k8 k10 --| |-- k7 k11 --| |-- k9 Vbsw --| |-- k5 k12 --| |-- k4 k13 --| |-- Vbsw k14 --| |-- k3 RCD --| |-- k2 Step --| |-- k1 Vcc --| |-- k15 ----------

k = display cathode outputs. Step = display scan clock from anode driver RCD = Reset Cathode Driver = start scan (from A&R chip) Note lack of ground pin -- runs between Vcc (8.2V) and Vbsw (4V).

Logic PCB --------- Firstly the discrete components. The 2 transistors above the C&T chip (28 pin) are the PSU. They, together with the pulse transformer, provide 3 voltages to the rest of the machine : Vss (+6V), Vgg (-12V) and Vcc (+8.2V)

The other 2 transistors, and associated components, form a schmitt trigger for the power-on reset (init) signal. This is fed to the C&T chip and the ROMs.

The little 8 pin chip :

1820-0855 Clock driver

---------- Phi_2 In --| U |-- Phi 2 Out Gnd --| |-- Gnd Vgg --| |-- Vss Phi_1 In --| |-- Phi_1 Out ----------

Buffers clock signals (at 6V levels) from anode driver to MOS levels for rest of CPU logic. From now on, Phi_n will mean the MOS signals!

The processor is in 2 chips, basically control (C&T) and data path (A&R). It operates on 56 bit words (which hold a normalised real number in BCD).

Control & Timing (C&T) -- the control part of the CPU (and keyboard scanner) 1820-0849

---------- Vss --| U |-- WS Phi_1 --| |-- Phi_2 --| |-- Gnd --| |-- Sync Init --| |-- IA --| |-- Carry --| |-- KC1 KR2 --| |-- KC2 KR3 --| |-- KC3 KR4 --| |-- KC4 KR5 --| |-- KC5 KR6 --| |-- Vgg KR7 --| |-- KR1 KR8 --| |-- IS ----------

KC = Keyboard Column, KR = Keyboard Row (to keyboard matrix) Carry = Carry flag input from A&R chip Init = power-on reset IA = Instruction Address = address to ROMs IS = InStruction = ROM Data input Sync = machine cycle sync signal Phi = 2 phase clock WS = Word Select from ROMs (used to update part of a 56 bit word)

Arithmetic & Registers (A&R) -- CPU data path 1820-0848

---------- Phi_1 --| U |-- Phi_2 Vss --| |-- Gnd IS --| |-- Vgg Sync --| |-- Disp4 WS --| |-- Disp3 Carry --| |-- Disp0 Data --| |-- Disp1 RCD --| |-- Disp2 ----------

Disp = Display data bus. RCD = Reset Cathode Driver. Phi = 2 phase clock, IS = InStruction (from ROMs) Sync = machine cycle sync signal Carry = Carry flag output WS = Word Select Data = bit-serial contents of X (display) register. Only used as T/P in the HP35

ROMs : These are 256 word (10 bit word) devices, serial address and data, in 10 pin TO99 cans (can't show in ASCII, so will show as 10 pin DIP).

1818=0024, 1818-0026, 1818-0028

---------- IS --| U |-- IA Init --| |-- Vgg WS --| |-- Sync Gnd --| |-- Phi_1 Vss --| |-- Phi_2 ----------

Signals as above (C&T chip)

Testpoints : There are 12 testpoints at one edge of the logic PCB, near the A&R chip. Used for factory testing/diagnosis. Just about _all_ signals are availabe either here or on the keyboard/display connector, so it's possible to find most faults from the signals here. The testpoints are :

o-- Vss o-- Gnd o-- Phi_1 ) Output from clock driver o-- Phi_2 ) o-- Vgg o-- IS o-- IA o-- Sync o-- WS o-- Init o-- Carry o-- Data

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