HP 35 internals....
--------------------

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