PDP12.ORG Logo

PDP-8 Minis
PDP-8/I
PDP-8 (0)
PDP-8/A
PDP-8/E (3)
PDP-8/F
PDP-8/I
PDP-8/L
PDP-8/M (2)
PDP-8/S (0)
Reference
PDP-11 Minis
PDP-11/R20
PDP-11/10
PDP-11/20
PDP-11/R20
PDP-11/23
PDP-11/34
Reference
PDP-12 Minis
PDP-12 Front View
PDP-12
Reference
Calculators
Front View of HP-9100
HP-9100B
Monroe 630
Tektronix 909
Test Equipment
Front View of HP-3440
GR-1683
HP-3440
Marconi 2955B
Miscellaneous
Front View of IBM 360 Front Panel
Help the Museum
About...
Contact Us
Hosted by KRTEN.COM

[PREV] [NEXT]

The Restoration of the PDP-8/I Minicomputer (PAGE 6)


The Mystery of the Deposit Command -- 2003 06 10 - 2003 06 11


Now that the front panel is complete (yes, I ended up using LEDs in the short term; but I have a quantity of official DEC PDP-8/I lamps waiting to be plugged in) I noticed that the LOAD ADDR and DEPOSIT switches don't quite work. The extended memory works fine with LOAD ADDR -- all the bits on the front panel correspond with the bits set in the switches. However, the regular (12 bit) addressing has some stuck bits, and (now, for the mystery) counts backwards when you hit DEPOSIT!

2003 07 05
I finally figured out what this was all about. Due to an existing problem where bits in the Accumulator are gated in to the adder logic, when the Accumulator had all ones in it, this would cause the "backwards counting". A bug, but still neat :-)

After the machine warms up for a while, it starts counting forwards. (2003 07 05 -- Turns out that it had nothing to do with "warming up" but just luck as to the contents of the Accumulator.) Another mystery is in the execute cycle. If I do a LOAD ADDR with 001/001 for the DATA FIELD and INST FIELD, and hit START, the RUN light comes on and it looks like it's trying to execute some program. However, using 000/000 for the two fields doesn't turn on the run light. I'm suspecting a memory problem with the first 4k of core, just off the top of my head.

It's at this point that I decided to exhaustively test all of the cards in the PDP-8/I.


Cards Tested -- 2003 06 29


Ok, so now I've tested all the single-height cards that were simple -- and things are working a little bit better. However, I have diagnosed that one of the M220 cards is hosed. The schematics on the web are for an M220A, whereas mine is an M220B. So, I'm busy drawing the schematics for the "B" version. This is an exhausting process of getting all the traces on both sides of the card into a text database, and then connecting all the wires in a schematic drawing package.

I cheated a little bit and scanned the card from both sides in my scanner, and then used a graphics package to mirror the solder-side. By printing that out, I was able to trace (with different colour pencils) the ground, +5V, and signal lines. Each line was entered into a text file (for example, "e1-15 e2-4 e3-7" to indicate that chip E1's pin 15 went to chip E2's pin 4 and chip E3's pin 7). Then, I used the existing M220A schematic as a base, and drew enough of the M220B schematic to get started testing.


Component Side View of the M220B Card S M L XL Solder Side View of the M220B Card S M L XL
These are the two pictures that I used to create the schematic. Of course, the original was only in black and white. Using a graphics editing package, I was able to "mask out" most of the colours, and all I was left with was the grey shade that corresponded nicely with the traces and the pins. Also, notice how the two pictures line up. If you're detail oriented, you'll notice that every feed-through hole has a designation of the form "X" and a number. This allowed me to construct the wiring listing without having to follow all the feed-through holes, I just treated them as if they were another component. I used different colours to indicate +5V (red), Ground (green), and signal wiring (yellow initially, but when I couldn't really see it I switched to orange).
[PREV] [NEXT]
Latest Updates
Master Reference
PDP-12 is alive!
New machine: PDP-8/L

Contact us This page was updated on Fri Feb 19 00:10:07 EST 2010 © 2000-2007 by Robert Krten.
All rights reserved.
Areas of expertise: 8044, 8051, ARC/CBS, ARCNet, ARM, ASM-86, Automated Disassembly, Avanza, Avionics Software, awk, bash, Bell 103, Bell 202, BITBus, Bootloaders, C, Call Processing, Caller ID, CF-UTTH, Clear Thinking and Common Sense, Client/Server, CMR-91, Code Analysis, Cold Standby, Computer Based Training, Course Developemnt, Course Presentation, CRS-1, curl, cvs, Czech Language, Databases, Data Acquisition, Data Recovery, Decompilation, Debugging, Distributed Processing, DMS-100, Documentation, Drivers, DSP, DTMF, E.164, Embedded Systems, Emulation, Equinox, expect, Fault Tolerance, FFT, Filesystems, Firewalls, Fractals, FreeBSD, FSK, Gammacell 40/1000/3000, Graphics, GSR-12000, Hardware, High Availability, Home Automation, Hot Standby, HTML, HTTP, IDA Pro, IIDS, Image Processing, In-Service Upgrade, Industrial Automation, Infrastructure, Internationalization, iRMX-86/286, ISBT-128, Kernels, ksh, Ladder Logic, Lexical Analyzers, Linux, Loggers, m4, make, Medical Devices, Memory Constrained Environments, Message Passing, Microcontrollers, MIDI, MIL-STD-2167A, NNTP, OpenBSD, Optimization, OrCAD, Oscilloscopes, PABX, Parallel Processing, Parsing, PDP-8, Photon, Porting, POSIX, POVRay, Powerpoint, PowerPC, Preprocessor, Process Control, Program Flow Analysis, Project Management, Prototyping, Public Speaking, QNET, QNX 2, QNX 4, QNX 6 (Neutrino), Radarsat, rcs, Realtime Techniques, Recursive Descent Parsers, Regression Testing, Regular Expressions, Requirements Analysis, Resource Managers, Restoration, Reverse Engineering, Scripting, Security Systems, sed, Serial Protocols, SGML, Simulation, SMDR, SMTP, Soldering, svn, SX-20, Synthesizers, System Architecture, T.4, T.30, TCL, TCP/IP, Technical Presentations, Technical Writing, Telecoms, Testcases, Testjigs, Text Processing, Threads, Tools, Traceability Matrices, TTL, UNIX, Utilities, UUCP, Validation Protocols, Vaulting, VAX/VMS, vi, Virtual Filesystems, Warm Standby, wget, Wirewrap, X-10, X.25, x86