• Please review our updated Terms and Rules here

My pdp-11/34 has an Easter Egg!

intabits said:
While repairing the M7859 board in my pdp-11/04, I learnt a lot about the workings of this board.

My pdp-11/34 has the later version of the M7859, and look what it does!


https://www.youtube.com/watch?v=_j0rE9StCs8
Click to expand...

Didn’t know of that one! But how dows it work? Where does it store the flag that tell it to start up showing the easter egg? There is as far as I know no non-volatile storage on the M7859, or is there on the newer version?

Or maybe some register did preserve its content since the power bounce was quite short? Would it make any difference if the power interuption is much longer?
 
You mentioned a couple of extra chips vs the older version. What are those chip types?
A real EEPROM seems unlikely.
If it truly is showing a non-volatile behavior, my guess is that there is a circuit on board that is designed to maintain a state for a relatively long time, such as a FET and a capacitor - implementing (for example) a single bit of memory that remembers for quite a long time without power.
Are there any caps on the board that are NOT obviously power decoupling caps?

Pete
 
On both variants of M7859 card the seven segment LED displays are driven by an SN7447A BCD to 7-segment driver IC.

Looking up the Texas Instruments datasheet for this device indicates that the 'P', the 'd', the '-' and the '_' characters are not displayable via this device. All of the other characters are ('1', '3' and '4').

It is conceivable for the 8008 microprocessor to display different characters in the same display digit (making use of persistence of vision) to construct other characters, but I find it difficult to remove segments to form the '-' and '_' using this technique...

Dave
 
The description of the 'older' card is here http://www.bitsavers.org/pdf/dec/pdp11/1134/KY11-LB_MaintMan.pdf along with the 8008 firmware listing.

The schematics of the older and newer cards are here http://www.bitsavers.org/pdf/dec/pdp11/1134/MP00015_KY11-LB.pdf and http://www.bitsavers.org/pdf/dec/pdp11/1134/MP00015_KY11-LB_Jan78.pdf.

The additional two ICs are E75 = SN7474 and E76 = SN7408.

There is an additional schematic (sheet 10 of 10) for the newer card revision (whereas the older revision of card only has 9 sheets).

The firmware ROMS have identical part numbers within the older and newer card revisions - so they must contain the same code.

On a cursory glance of the 8008 source code, I can't see an Easter Egg...

Dave
 
daver2 said:
On both variants of M7859 card the seven segment LED displays are driven by an SN7447A BCD to 7-segment driver IC.

Looking up the Texas Instruments datasheet for this device indicates that the 'P', the 'd', the '-' and the '_' characters are not displayable via this device. All of the other characters are ('1', '3' and '4').

It is conceivable for the 8008 microprocessor to display different characters in the same display digit (making use of persistence of vision) to construct other characters, but I find it difficult to remove segments to form the '-' and '_' using this technique...

Dave
Click to expand...

The 7447A decoder is actually resident on the KY11LB front panel board. Inputs A,B,C go back to the M7859. Input D is grounded. So only digits 0-7 and blank can be displayed.

The only way this could work would be to hack the front panel and allow the segments to alternately be driven via a different logic path.

Not an Easter Egg. More like an April Fools Joke.
 
Forgot about the ‘D’ input being grounded, so yes, only 0..7 and the digit not being enabled at all to give us the blank.

Always better to ‘debunk’ with evidence rather than just stating ”April Fool”...

An interesting diversion in an otherwise boring day (and I learnt something new to boot).

Come clean :).

Dave
 
Also not sure it is possible for the micro on the M7859 to distinguish that it is connected to an 11/34 vs an 11/04 ...
 
Got me too, even after someone posted a 1-Apr warning...
I think you guys that have this HW should get busy and implement it, using the various theories presented here. :)

Pete
 
Probably the simplest would be to bury a small Arduino module inside the KY11LB console module and connect it to the LEDs and some of the front panel buttons and have it play with the display drive lines (6X active low) and segment lines (7X active low).
 
You must log in or register to reply here.
Back
Top