Subject: HP9915

From: [email-masked] (Tony Duell)
Date: Sat May 3 19:08:00 2003

I've recently had an HP9915 on the bench. This, as many of you are aware, 
is related to the HP85, but it's in a metal case with no printer, 
keyboard (although there is a keyboard connector on the back) or monitor 
(again, there's a composite video output on the back).

Some details of the insides :

Almost all the HP ICs are the same as those in the HP85 (CPU, CRT 
controller, RAM controller, Keyboard controller, ROMs, I/O buffer, even 
the printer controller is there). Much of the rest of the circuitry is 
similar to that in the HP85 (PSU, etc). 

There's an extra 8048 microcontroller that handles the power-on reset, 
self-test and autostart keys, and the front panel LEDs. It connects to 
the HP CPU via an I/O Translator IC, which is the same as the one used in 
the HP85 interface modules. Also linked to this microcontroller is a PCB 
that can contain up to 8 2716 (or 2732 if you rearrange the links) 
EPROMs. I have no idea (yet) how to format data to put in said EPROMs.

The large board in the bottom of the case contains the PSU, and much of 
the logic circuitry. There are 6 'option ROM' spaces at the back of this 
board, identical to those in an HP85 ROM drawer. 

The PSU provides the same voltages, in much the same way, as the HP85 CPU.
The output of the transformer is rectified and fed t oa switching
converter (U30, 3524 is the controller IC) to provide +12V, -5V and -12V. 
This poweres a second switching converter to provide +5V. The +6V is
obtained by a linear regulator. There is a crowbar (Q4) which will short 
out the input to the first PSU if the +12V line rises too high. This will 
then blow the mains fuse (something to check if the mains fuse blows at 
switch-on -- maybe Q5, PIC645, the first chopper is shorted).

Another PCB plugged into the left edge of the main PCB contains the video 
RAM, printer chip (used here for the timers, I suspect), buffers/clamp 
networks for the keyboard and control connectors, a state machine to 
generate the video sync signals, and the composite video mixer. It has 
been suggested this is the 'operator interface' mentioned on the options 
box on the back, but I think not. I think this must be a standard part of 
the machine, and that the 'operator interface' is the keypad on the front.

Talking of the keypad, the top 4 keys are connected as the k1-k4 keys of 
the HP85 keyboard. The blue key is connected as the shift key.

On the back of the machine (actually on the vertically-mounted PCB that I 
mentioned a couple of paragraphs back) are 2 D connectors. The pinouts 
seem to be : 

Keyboard (DB25)
1 : chassis ground
2 : R0X (this is Row 0 output, buffered)
3 : R1X
4 : R2X
5 : R3X
6 : R4X
7 : Logic ground
8 : Logic Ground
9 : R5X
10 : R6X
11 : R7X
12 : R8X
13 : R9X
14 : C0X (Column 0 input, buffered)
15 : C1X
16 : C2X
17 : C3X
18 : C4X
19 : C5X
20 : C6X
21 : C7X
22 : KSX/ (Shift key, connect to logic ground to shift)
23 : KCX/ (Control key, ditto)
24 : KCLX/ (Caps lock key, ditto)
25 : SpkrX (Speaker output, return to logic ground)

Control connector, DA15
1 : LED0X (High if top LED on frontpanel is on)
2 : LED1X
3 : LED2X
4 : LED3X
5 : LED4X
6 : LED5X
7 : LED6X
8 : LED7X
9 : RunX/ (RUN LED output)
10 : TestX (selftest LED output)
11 ; PWOX (Power OK output)
12 : Logic ground
13 : KeyStartX/ (ground for autostart key)
14 : KeyStX/ (ground for self test key)
15 : Chassis ground.

The keyboard matrix would appear to be the same as that in the HP85

OK< a couple of questions 

1) What is the impedance of the speaker in the keyboard (presumably 
between SpkrX and logic ground? 80 ohms?

2) What is the OD of the tape drive roller? Mine has decayed (what a 
suprise), and I need to rebuild it. It should be the same as the roller 
in the HP85, the HP9825, HP9815, etc....

-tony