HP 9872T Trouble Shooting

[husmann]

Last week I obtained a HP-9872T plotter in very good condition. In my dreams it will be controlled by a HP-41 (Plot Module, HP IL module, HP IL/HP IB interface) -- some day.

Unfortunately the HP-9872T does not initialize correctly when powered on (see short video http://www.huswelt.de/MoHPC/HP9872T_PowerOn.mp4).

Does someone know how to fix this, or knows someone who knows… Any hint is highly appreciated.

Thanks in advance.

Wolfgang

[Paul Berger (Canada)]

There is a service guide for the 9872C/T at hpmuseum.net along with user manuals.

[husmann]

Thanks for your reply.

(06-09-2018 08:56 PM)Paul Berger (Canada) Wrote:  There is a service guide for the 9872C/T at hpmuseum.net along with user manuals.

I have found no mention of my plotters behaviour neither in the “Operating and Programming Manual” nor in the “Service Manual”.

The “Confidence Test” cannot be started: the plotter simply ignores flipping the test switch on the rear panel to 1.

The internal red push button (reset) sometimes stops the noise (at least as long as the red button is pushed) but the self test cannot be continued because the green push button (continue) is ignored.

Help! ;-)

Wolfgang

[cruff]

I have two 9872C plotters, which are very similar, as you probably know. One of mine was stored where one or more mice took up residence, chewing one of the multi-conductor cables which carried the signals from the limit switches. On power up, the initialization sequence would cause the carriage to run to the extreme +Y direction and made a horrible noise until powered off.

If you haven't opened it up to check, you should do so to verify lack of cable damage, that there are no obviously bad capacitors. If you have the service manual, verify all supply voltages are correct.

[husmann]

Thanks for your reply.

(06-09-2018 11:48 PM)cruff Wrote:  … verify lack of cable damage, that there are no obviously bad capacitors. If you have the service manual, verify all supply voltages are correct.

I looked for cable damage, looked for leaking or swelling capacitors, checked every fuse and verified every supply voltage. Everything seems to be ok, no mice ;-)

BTW: The noise comes from the y-axis motor: if I turn off the y-axis motor (switch “S2 Y MUTE”) the noise vanishes. Unfortunately the plotter shows the same behaviour as before: front panel LEDs “ENTER”, “OUT OF LIMIT”, “ERROR”, and “CHART LOAD” are on and no self test can be started.

Wolfgang

[rprosperi]

Try joining then posting questions on the the Vintage HP Computer Systems group, found here: https://groups.io/g/VintHPcom.

The group is fairly active, and likely your best chance of finding someone with experience with one of these plotters.

Good Luck.

[cruff]

(06-10-2018 02:11 PM)rprosperi Wrote:  Try joining then posting questions on the the Vintage HP Computer Systems group, found here: https://groups.io/g/VintHPcom.

The group is fairly active, and likely your best chance of finding someone with experience with one of these plotters.

I'm a member of that group also, I'll try to help out either place.

What happens when you follow the self test steps in the service manual section 2-23? Note that the self test switch in this section is internal, not the one on the back panel.

[husmann]

(06-10-2018 02:11 PM)rprosperi Wrote:  Try joining then posting questions on the the Vintage HP Computer Systems group, found here: https://groups.io/g/VintHPcom.

Thanks for that hint. My posting is waiting to be approved by the moderator of VintHPcom.

Wolfgang

[husmann]

(06-10-2018 04:29 PM)cruff Wrote:  

(06-10-2018 02:11 PM)rprosperi Wrote:  The group [VintHPcom] is fairly active, and likely your best chance of finding someone with experience with one of these plotters.

I'm a member of that group also, I'll try to help out either place.

What happens when you follow the self test steps in the service manual section 2-23? Note that the self test switch in this section is internal, not the one on the back panel.

Thanks a lot for your offer. Here we go:

[2.23.a] Remove power from the plotter. ✔︎
[2.23.b] Remove all interface cables. ✔︎
[2.23.c] Open the plotter. ✔︎
[2.23.d] Apply power to the plotter. The plotter powers up with front panel LEDs “ENTER”, “OUT OF LIMIT”, “ERROR”, and “CHART LOAD” on and a y-axis motor buzzing very loudly (see http://www.huswelt.de/MoHPC/HP9872T_PowerOn.mp4).
[2.23.e] Set the self test switch S4-2 to the on position. I assume the on position is towards the front panel away from the fan. Setting switch S4-2 to the on position has no effect on front panel LEDs and y-axis motor.
[2.23.f] Press the Reset switch S5 pushbutton (red). While the reset button is pressed the y-axis motor stops buzzing but the front panel LEDs stay on. Releasing the reset button has no effect on the front panel LEDs but in most cases the y-axis motor starts buzzing again. Whether the y-axis motor is buzzing or not has no effect on the following results.
[2.23.g] The Self Test LEDs will flash sequentially as a lamp test. Total failure here: each and every self test LED stays off.
[2.23.h] Press the Continue switch S3 pushbutton (green). ✔︎
[2.23.i] The test will run … Even after waiting more than two hours the front panel LEDs stay on, the y-axis motor continues buzzing loudly (if not silenced in 2.23.f), and all self test LEDs stay off.

Thanks for your efforts.

Wolfgang

[cruff]

(06-10-2018 08:05 PM)husmann Wrote:  [2.23.g] The Self Test LEDs will flash sequentially as a lamp test. Total failure here: each and every self test LED stays off.

This means the BPC microprocessor is not executing instructions properly. Do you have an oscilloscope or logic analyzer available to help with the diagnosis? Fortunately the logic levels should all be 5 V.

If ROMs U55 and U56 are socketed, try reseating the ROM chips. Perhaps some pin/socket contact oxidation is interfering.

Can you observe the clock signal at pins 5 and 6 of clock divider U60? They should be 5 MHz with alternating levels.

[husmann]

(06-10-2018 09:37 PM)cruff Wrote:  This means …

Thanks a lot for all your hints. I’ll need some time to check them all. As soon as I have completed them I’ll post my results here.

Wolfgang

[husmann]

(06-10-2018 10:49 PM)husmann Wrote:  Thanks a lot for all your hints. I’ll need some time to check them all. As soon as I have completed them I’ll post my results here.

Finally I managed to have a few minutes to return to my sick mechanical pet. My results:

• reseating ROMs U55 and U56 had no effect

• photos of the clock signal at pins 5 and 6 of clock divider U60 can be downloaded at http://huswelt.de/HP-9872T/HP-9872T.zip - the photos are taken from a borrowed simple oscilloscope

Thanks, Wolfgang

[husmann]

This thread is continued in the Vintage HP Computer Systems group (https://groups.io/g/VintHPcom).

Thanks, Wolfgang

[husmann]

I’m giving up. After having spent (again) quite a few hours analyzing the defective electronics of my HP-9872T I come to the conclusion that it is time to withdrawal. I do not have neither the equipment nor the time budget to continue this quest. Thanks to all who helped me.

If someone has interest in this HP-9872T: I give it away for free – complete with one spare paper roll, the service manual, the operating and programming manual, and the quick reference guide (no cables, no pens, 240 V AC).

[vassilisprevelakis]

After many many years, the riddle has been solved.

I strongly believe that there is nothing wrong with the printer itself, but the program ROM has failed.

The 9872C and its companion 7220C have a 16bit multiplexed address/data bus, so it needs two 8-bit x 8096 ROMs to store the control program (actually the 7220C has two more ROMs as it needs an additional 2K words of program memory). Both plotters have a very similar architecture and internal layout, but the main PCB is different (one has the HP-IB interface, the other has RS232). The ROMs are predictably different as well, but as we shall see not *very* different.

In the problematic 9872C, the MSB ROM is totally flaky. For some locations it returns different values each time it is read, and for others the returned value may be corrupted.

I am looking for the binary images of 9872C ROMs to replace the failed ROMs.

In the mean time I had a look at the code , comparing the code in the 7220C ROMs with what I could read from the 9872C ROMs.

The CPU is a custom HP processor (predictably) consisting of the BPC unit from the famous hybrid processor used in the 9825 calculator. The part in the two plotters is identified as the 1818-2500 and it has the 15-bit version of the BPC. Fortunately the manual for the NMOSS-II hybrid is available on the bitsavers archive, so I could write a quick and dirty disassembler for the BPC.

The BPC uses the first 32 words of memory (0000 - 001F) as "registers", so the code starts at address 0020 hex.

Looking at the 7220C startup code

Code:


0020  F060    CMA                   ; two's compliment of A-Register
0021  0081    LDA 0081           ; 
0022  3016    STA Reg2           ; Register 22 which is the Serial interface Register
0023  2081    ADA 0081           ; so I am guessing it is initializing the serial i/f here
0024  3016    STA Reg22
0025  2081    ADA 0081
0026  3016    STA Reg22
0027  2081    ADA 0081
0028  3016    STA Reg22
0029  7C82    SSS 002B          ; here is the interesting bit, it looks for the state of the test switch
002A  E97A    JMP 017A,I        ; and branches to the 'normal' code
002B  69B5    JMP 01B5           ; or the test code (blinking the diagnostic LEDs)

and here is the code I extracted from the broken 9872C ROMs (note that this code is corrupted).

Code:


0020  FC60    SBP 0020           ; <------- skip if B-Register is positive
0021  0084    LDA 0084
0022  3016    STA Reg22         ; Register 22 which is the HP-IB interface Register
0023  2084    ADA 0084
0024  3C16    STB Reg22
0025  2084    ADA 0084
0026  3016    STA Reg22
0027  2084    ADA 0084
0028  3C16    STB Reg22
0029  7C82    SSS 002B          ; here it also checks the state of the test switch
002A  E8D5    JMP 00D5,I
002B  E8D7    JMP 00D7,I

So the code looks more or less the same, initializing the HP-IB control register and checking for the state of the test switch. But in this case we see that the instruction at 0x0020 has changed from a CMA to an SBP 0020 which is a conditional branch instruction. If the contents of Register B are non-zero (i.e. bit 15 is zero) then jump to address 0x0020. Voila! Since the BPC has been reset, the B Register is zero and the SBP 0020 jumps to ... 0020, i.e. to itself, looping for ever.

Great, so the printer stays in reset state with large chunks of its logic uninitialized, which is probably what causes the Y-axis motor to hum loudly.

I am waiting for some parts to allow me to swap the boot ROMs with EEPROMs so that I can carry out more testing of the unit. Ultimately I hope to be able to load working code into the EEPROMs and hopefully this will get the 9872C working again.

Keep fingers crossed.

Vassilis

http://www.series80.org

[cruff]

By the way, the 9872C schematics seemed to show a discrepancy with the mapping of some of the address bus bits to the ROM address pins. I'm not sure if this is real or not, as it also showed the ROM enable signal (labeled as "F1") going to an address pin on the 36000 ROM instead of the CE pin.

[vassilisprevelakis]

(03-01-2023 01:28 PM)cruff Wrote:  By the way, the 9872C schematics seemed to show a discrepancy with the mapping of some of the address bus bits to the ROM address pins. I'm not sure if this is real or not, as it also showed the ROM enable signal (labeled as "F1") going to an address pin on the 36000 ROM instead of the CE pin.

Yes they got the pins mixed up in the schematic. The connections are correct in the 7220C schematics. On the actual device the connections are (of course) correct, or it wouldn't work even with a functioning ROM :-)

Nevertheless, on the 9872C they got the screen labels wrong for the +5V and Common test pins, next to U28. They are actually reversed which caused some early testing confusion on my part as I was unlucky to use these test points for my initial connectivity testing.

As an aside, although the 9872C and then 7220C were identically priced, the RS232 model had additional functionality (ability to generate arcs and circles) plus more ROM and expandable RAM.

Finally, the 7221C which uses a compressed version of graphics language instead of HP-GL, can be turned into an 7220C by simply replacing the 2 8K ROMS with those of the 7220C.

If anybody wants the ROM dumps of the 7220C, I have uploaded them to the series80 website.

These are ASCII files with lines starting with the hex address followed by 32 bytes of data encoded as 2-digit hex numbers.

I have also uploaded the combined file with lines starting with the address in hex followed by 16 words of data each encoded as 4 digit hex numbers.

U55: https://www.series80.org/Misc/7220C-U55-MSB.hex
U56: https://www.series80.org/Misc/7220C-U56-LSB.hex
And the combined MSB and LSB ROMs is at:
https://www.series80.org/Misc/7220C-COMBINED.hex


Vassilis
http://www.series80.org