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DEC PDP 11/40 Project


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  DEC PDP 11/40 Project by Bill Degnan - 03/11/2008 16:08
This weekend my wife and I drove 1200 miles to pick up a working PDP 11/40, RL02 disk drive, and Decwriter II from A. Carder in South Carolina. He's an expert on the PDP 11/40 and runs this web site:

Here are the initial pics before I started working.

  PDP 11/40 Bootstrap and Decwriter Term by Bill Degnan - 03/12/2008 21:55
1) Assuming that the cable with a sticker labeled "11/23 DBT" with the 44 pin female card port connector connects to the M7856, will I hurt anything if I plug it in backwards? Is there a way to tell which way is correct? I assume the end with the label showing / facing out would be correct, but ...

probably nothing *good* will happen, disconnet if you don't get the desired
results quickly.


2) On the other end of this cable there is a male 25-pin connector. This cable has a sticker that says "DBT." I assume I should attach this end to the Decwriter II, but because the terminal's interface cable is also has a 25-pin male, I need a female/female gender changer to make the connection, correct?

Open the chassis in the back and the box that covers the the cable connector. Disconnect what should be the other end of the RS 232 cable. Plug the cable already connected to the M7856 instead. If you can't do that, then get a straight-through genderchanger 24-pin female/female

[FOLLOW UP NOTE - The cord is soldered into the terminal from the inside, I had to use a straight through gender changer]

3) Is the cable referred to in questions 1&2 also a display cable for use with a DEC terminal? (In which case I have to change the baud of the M7856 but don't need a gender changer?)

YES - A printer terminal is the same as a display terminal as far as the PDP 11/40 is concerned. Just change the baud rate on the M7856 from 300 baud for the printer to ??? to match the terminal

4) I understand that 165144 is the memory location to boot the disk drive. For using the Decwriter terminal do I need to load a specific address, or will the ROM ignore the front panel, and automatically boot (show a @ upon booting)?

WRONG - 165144 is the memory location for the bootstrap program stored in ROM on the M9312, once you see the @, the command to access the drive is DL0 (or DL1,2,etc)

  PDP 11/40 Testing with Toggle Swich by Bill Degnan - 03/13/2008 07:58
Before I started with the terminal, I attempted to test just the computer. I was not able to get the test program (below) to work. I have some vague ideas about what's wrong, but I am inexperienced with these systems I need to determine whether I am toggling in the test program incorrectly, or if there is possibly a hardware issue.

1. verified cards were inserted correctly...

(I have since then straightened the 7856.

7856 taking up slots below the M981 (should the 7856 be in the same row of slots as the UNIBUS card?)
67273 next to 7856
M791 RAM - green led lights up
M9312 in top slots on end

2. test "light chase" program
power on

halt / start / enable (* see below)

toggle in 001000 and depress load addr
toggle in 005000 and uppress dep
toggle in 005200 and uppress dep
toggle in 006100 and uppress dep
toggle in 000005 and uppress dep
toggle in 000775 and uppress dep

toggle in 001000 and depress load addr
depress start

-Rather than the expected chasing light pattern, the following lights display:
Address: 0000006
Data: 0/074014
RUN = a dim light
BUS = a bright light
PROC = a bright light

I assume that each time I uppress "dep" the address byte increments by one and the values are being stored correctly. It seems like the data is being stored in memory. I am looking for toggle oriented diagnostics.

Is my toggle-by-toggle is being entered correctly or am I missing a step? I am attempting to get a handle on the variables.

For kicks I tried to attach the computer to the decwriter, but this did not work. I'll come back to that after I am sure the computer itself is ok.

(*) The PDP-11/40 system manual states that a person should depress the HALT switch before powering on the system. The dim RUN light is normal when the system is running, per the manual.

  PDP 11/40 Progress by Bill Degnan - 05/28/2008 21:53
Moved the 11/40 into the guestroom of my house from the shed. Now it will be in an air conditioned environment for the summer.

I was able with the help of Herb Johnson to properly re-insert the 64K memory card. With this task complete, the system works reasonably well. Previously the card was not fully inserted and that was the cause of problems.

Here is a link to Herb's PDP 11 web page:

I have been able to save data into memory and then recall it. I was also able to toggle in the "chase the lights" program.

Change the baud rate on the serial card in order to use the Decwriter LA36 terminal. The LA36 I believe has been wired for 1200 baud, someone must have upgraded it from the original max baud of 300. More details and pics to follow.

  PDP 11/40 Memory Map by Bill Degnan - 05/31/2008 17:18
So, what's on this thing?

1st - It has a 64K Memory Card

According to the Bootstrap Loader on page 3-24 of the 11/40 system manual ...hmmm...not sure exactly.

I was kind of hoping that there would be something at loc 165144, but there is no code to bootstrap at this location. I did find code at 777574, not sure what it is.

more later.

  Card Inventory PDP 11/40 by Bill Degnan - 06/01/2008 01:23

empty 1 -- (IF PRESENT WOULD BE USED FOR A 7239)
empty 2 -- (IF PRESENT WOULD BE USED FOR A 7298)
7232 3 A-F (U WORD)
7231 4 A-F (DATA PATHS)
7233 5 A-F (IR DECODE)
7235 6 A-F (STATUS)
7236 7 A-F (TIMING)
672A 9 D (GRANT CONT.)
M7856 10 F-C (RS232 WITH 19.2 OPTION?)
G7273 11 C-D
7891 12 A-F (64k MEMORY)
M9312 13 A-B (ROM CARD)
672A 13 D (GRANT CONT.)

I am starting to work further and I wish to explore the terminal interface using the M7856. Unfortunately things have been slow because I don't have the M7856, 7891, or M9312 docs (still trying to find). I have the original bootstrap ROM that came with the M9312 still present as it was originally. When I load the address 165144 and examine (I was told this was the address to activat e the ROM), nothing appears to be loaded into memory (just blank RAM: FFF FFF FFF FFF/ 000 000 000 000 etc.) at 165144. When I attempt to activate the ROM (halt, load address 165144, depress enable, and then start). There is no change to the data in memory location 165144.

Should it be 765144 because of the memory management card (7236)? Whatever the case I do not get a response from the terminal. I understand from my notes of the conversation I had with the orignal seller that to use the LA 36 DecWriter II with the 11/40 I will have to change the baud rate jumpers or switches on the M7856 from 19.2K to a lower setting for the terminal. So, for the time being I am using a computer with the windows terminal program set for VT 52, 7 bits no parity, and 19.2K baud. I have also tried other settings such as 8 bits no parity, VT 100, etc. I am confident that the computer is looking at the correct COM port.

Knowing what I know, I would not expect any terminal output, because there is no program loaded.

I have worked hard to find answers and docs on my own, but I am at a stand still at the moment. Any tips are appreciated.

NOTE: There's a note on the DEC terminal to indicate that it was upgraded to 1200. In local mode the DEC terminal works OK.

  Video Simple Counter by Bill Degnan - 07/27/2008 21:02
Video of my progress is found here:

I have found that it's nice to put the bootstrap in as high of a location as possible, but I see no reason to believe that it's a *requirement*, so I will install it where ever I can save it, and see how that goes.

  Bootstrap RX02 drive to the PDP 11/40 by Bill Degnan - 08/29/2010 17:30
RX211 Bootstrap Loader

Location Contents
002000 012701
002002 177170
002004 012700
002006 100240
002010 005002
002012 012705
002014 000200
002016 012704
002020 000401
002022 012703
002024 177172
002026 030011
002030 001776
002032 100440
002034 012711
002036 000407
002040 030011
002042 001776
002044 100433
002046 110413
002050 000304
002052 030011
002054 001776
002056 110413
002060 000304
002062 030011
002064 001776
002066 100422
002070 012711
002072 000403
002074 030011
002076 001776
002100 100415
002102 010513
002104 030011
002106 001776
002110 100411
002112 010213
002114 060502
002116 060502
002120 122424
002122 120427
002124 000007
002126 003737
002130 005000
002132 005007
002134 000000



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