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  PDP 8e by Bill Degnan - 01/31/2010 18:15
Started work on a PDP 8e full-height rack with DecPak RK05, Decprinter-I, and VT-52 terminal.

Digital PDP/8e Computer


Status: System itself appears to work, in that I can load programs from the toggle switches and do some simple terminal I/O. There is an issue with the disk drive controller, but I will write more about that later. Looking for best way to use a laptop as a terminal to simulate a papertape for program I/O.

Printer has not been tested.

  Diagnosing M7104, M7105, M7106 by Bill Degnan - 02/15/2010 16:43
I am not sure which drive controller is bad. [NOTE: I learned later that none were bad.]

- system does not reset the drives when SW switch is moved [NOTE: I learned later that the bootstrap card for the drives is bad]

- does not seem to reset the flag that indicates it's done with the read of the drive.

URL to manuals

M7104 RK05 Disk Controller 1 (data buffer register and status)

M7105 RK05 Disk Controller 2 (major registers)

M7106 RK05 Disk Controller 3 (control module)


  PDP 8/e Configuration by Bill Degnan - 02/15/2010 17:10
A listing of the cards installed in my PDP 8e, from front to back.

PDP 8e card cage. Click image for larger image.

M8330 - CPU 1
M8300 - CPU 2
M8310 - CPU 3

M837 - Time Share and for memory extention beyond 4K

M8650 - terminal I/O

M7104 - Controller 1 for RK05
M7105 - Controller 2 for RK05
M7106 - Controller 3 for RK05
(these have an "over the top" controller cover for all three of these drive cards)

M882 - Clock card

M847 - Bootstrap loader for RK05

M935 - Unibus Bridge

M849 - Shield for RF interference

G111 - 8K Core (there are 4 8K core cards in this system = 32K)

8320 - bus terminator

  PDP 8e Front Panel Intro by Bill Degnan - 02/15/2010 18:46
Reading the Lights on the front panel.
Detail image of PDP 8/e Front Panel in new window

State - ?
Status Flags (F, D, IR0, IR1, IR2, MD DIR,etc. values)
AC - accumulator
MD - memory / data *
MQ - Multiplier quotient
BUS - What's going to the BUS.

* MD is the swtich setting most used, this is the what tells you the values in memory.

1. Press HALT, move the dial to "MD"
2. Toggle in the memory location in octal. for example: (00)0 200 = all switches down except #4.
3. Press LOAD ADDR to tell the computer that 0200 is the address you're interested to view its contents. The lights should match.
4. Press EXAM to view the data stored in 0200. The computer will display the whatever data is in 0200.
5. Press EXAM again to see what's in the next memory location (0201)...

Perform steps 1-3 above (assuming you want to enter something in 0200), but do not press examine.
1. toggle in the octal value you want to save in location 0200. For example 7001 would correspond to 0,1,2,11 up and the rest of the switches down.
2. Use the depress switch (upwards) to save the data in the memory location.
3. The computer automatically moves to the next location in memory (0201). You can then deposit the next data, etc.

Here is a quick review of how to access and test extended memory on a PDP 8, assuming you have the M837 card installed.

On the PDP8 the EXTD ADDR LOAD is used to address memory above the 4K boundary.

000 200 = octal address w/i 0 page
070 200 = octal address w/i 7th 4K page (28K)- To access these kinds of addresses you need to use EXTD ADDR LOAD first. Note: you set the data and program locations separately.

1. Use switch register 6,7,8 to pick octal 0-7 - program location

2. Use switch register 9,10,11 to pick octal 0-7 data location

Let's say you want to load a program in 7th 4K RAM and also to write your data to the 7th 4K RAM area, here is what you do:

6-7-8 up = 111 = 7 octal = 28k page for program location to be saved

9-10-11 up = 111 = 7 octal = dave my data in the 7th 4K page = 28K area

After you have toggled in your memory locations, depress EXD ADDR LOAD to tell the system that you're going to put your program in extended address (0)70 000 (28K area)

Now simply return to "normal" operation as if you were using lower addresses. For example if you put all toggle switches down and click LOAD ADDR you'll be loading 070 000 rather than 000 000.

  Booting a PDP 8/e with OS/8 RK05 drives by Bill Degnan - 02/15/2010 23:35
There is a specific order for booting the computer. First put your OS/8 diskpack in drive 0.

A pair of Digital RK05 decpac drives. Click thumbnail for larger image.

IF you have an M847 bootstrap card:
1. Power on the drives first. On my system that involves flipping the breaker of the power supply up.
2. Wait until drive sets itself up.
3. Power on PDP 8 using key.
4. On the RK05 PRESS load DOWN (there should be no disk in the drive). Open the door and slide the disk slowly into the RK05.
5. Push RUN switch up. Watch for lights PWR/RDY/ONCYL to come on.
6. On PDP 8
-halt down
-SW down
- halt up
- lift sw up and down
- see response from drive
7. Put switch of drive back to load when drive not in use - this is the equivalent of taking the drive offline. When load list comes on after a few seconds you can then remove disks.

IF you do not have a bootstrap card:
Toggle in the following program to manually bootstrap your RK05:


  Problem with Switch Register 5 by Bill Degnan - 06/30/2010 21:09
Unfortunately now when I toggle programs into my PDP 8e the 5th switch register data address cannot be set to 1 at any address. When I toggle the switch register 5 to the on position and then up-press deposit it no longer lights up. This makes it hard to load diagnostics, etc.

  Not a problem with switch register by Bill Degnan - 07/03/2010 08:25
The problem turns out to be a dead lightbulb in switch register 5. I can set a data value that uses the 5th significant digit ie.

000 111 000 000

the lights say

000 101 000 000

but the value is there. whew.

Here is a source for lightbulbs:

  You Tube Video PDP/8e by Bill Degnan - 07/28/2010 09:10
Today I was able to boot the system into a stable OS/8 Session. I loaded Adventure and BASIC from disk. I was able to write and run simple programs from BASIC.

The manual bootstrap loader for the RK05 is
0030: 6743
0031: 5031

Video (from Android phone)

VT-50 DECScope terminal displays the inventory from Adventure! game.

  PDP/8e Progress by Bill Degnan - 07/28/2010 14:22
Last night David Geesw. stopped by after work to take a look at my PDP/8e set up. I told him that I was having trouble interfacing with a serial terminal. He helped confirm that indeed the two M8650 serial card(s) I have are set to 9600b 8N1, and that they're both not fully functional. We ran a series of tests to come to this conclusion. It's possible to do a little terminal I/O, but suppose for example you depress the E key a bunch of times, you'll see on the screen (for example):

We also learned that the M847 - Bootstrap loader for RK05 also bad. We ran the 2 line bootstrap to manually boot the disk drive.

The good news
- The drives work, and the drive controllers work.
- The CPU seems to be OK.

  PDP-8e Running OS-8 by Bill Degnan - 08/19/2010 23:39

I purchased a replacement M8650 serial card. Although it was highly customized I was able to restore it to a stock configuration. I replaced the 14hz crystal with a 19.661 crystal and upgraded the card to 9600 baud. The system now boots up with no problem and I have begun learning OS-8, the operating system for the PDP 8 series. To the right is an 11/05 system. On the display terminal is a directory listing of the drive. OS-8 boot screen

The command to load BASIC in OS-8 (with partitioned RK05 drives)

The prompt used by OS/8 is the period ( . )

OS/8 has to partition RK05 disk packs, so you need to remember that when trying to run a program. For example, to run a directory of the A partition in drive 0 type the following command:

.DIR RKA0 [return]

The B partition of drive 1 would be:

.DIR RKB1 [return]

  Toggle-in Code the Lights PDP 8e by Bill Degnan - 10/25/2011 23:41
Here is the program I use to increment the AC. I have seen this elsewhere done differently, but this will work:


This is a simple test program for your PDP 8e to verify basic functionality.

  Names for Partitioned RK05 drives by Bill Degnan - 03/18/2012 07:34
The command for listing the directory of partitioned RK05 drives (OS/8)

.DIR RKA0 (first half of first disk)
.DIR RKB0 (2nd half of first disk)
.DIR RKA1 (first half of 2nd disk)
.DIR RKB1 (2nd half of 2nd disk)

if you run .DIR withouth parameters, I believe it will default to the first partition of the first disk.

To run a program (ending in .SV) then use this pattern:

.RUN RKA0:BASIC - for example, if BASIC.SV is located on the first partition of the fisrt RK05 disk pack.

If OS/8 gets out of whack/irresponsive you can get back to the dot prompt by stopping the machine, setting the program counter to 7600, and START.



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