The PDP-8 Family had a series of different Point Plot Display Systems.
They were all similar, but not program compatible.
The basic idea was that two D/A converters were used to generate a deflection
voltage that was applied to an oscilloscope.
The deflection voltages defined a location on the oscilloscope screen.
Once the location was determined a pulse was sent to the oscilloscope to illuminate
By rapidly illuminating points on a 512x512 or 1024x1024 point grid,
any pattern could be displayed.
The AX-08 point plot control was part of the LAB-8.
The D/A converter was limited to 9-bits and so the display grid was 512x512.
The LAB-8 interface included a A/D converter that used parts of the display logic and
a discrete input register for switches.
The discrete input was used as part of the game interface for spacewar.
The VC8/I point plot control is similar to the LAB-8 version, but simplified.
The D/A converters are now 10-bit, so the display grid is 1024x1024.
There is no A/D and there is no discrete input capability.
There is a light pen interface, and the ability to interrupt the processor
based on the light pen status.
The VC8-E point plot control is different again.
The earlier controls assumed the displays were fast,
the deflection time would be less than an instruction time, and
therefore a delay was not necessary to allow the beam to settle.
The VC8-E had a flag that indicated when settling was done and the point could be displayed,
so it was able to use a display that had a longer settling time.
Initially, there is no control over the intensity of the point,
although this was changed in later revisions of the option.
VC8/I Using Standard Flip Chips
I have a handful of Flip Chips; a two row, 32 slot backplane; and a KA8-E; so I thought I might try to
duplicate a VC8/I.
The KA8-E (M8350) is a Positive I/O Bus Interface that is equivalent to the PDP8/I Posi-Bus.
The interface allows OMNIBUS processors to connect to Posi-Bus options.
The Posi-Bus can be divided into 4 sections: the Buffered Memory Bus (BMB), the Buffered AC Bus (BAC),
the AC Input Bus (AC In), and the control bus.
The BMB presents an instruction to the I/O device controller.
The control bus issues I/O Pulses (IOP) based on the I/O instruction being executed.
Each I/O instruction can have 3 IOP: IOP1 if bit 11 is set, IOP2 if bit 10 is set, and IOP4 if bit 9 is set.
The pulses are generated sequentially during an instruction and trigger events within the device.
The BAC contains the AC contents for transfer to the I/O device.
AC In provides a return path for data to the AC.
I chose to include the essentials of the VC8/I: a 10-bit X,Y location, and a 3 level brightness control.
The light pen interface and interrupt logic was omitted.
The design summary can be found here.
I very much doubt anybody will replicate this, but if they do, let me know and I will try to help.
For display, I initially used a portable technicians
oscilloscope that I modified for intensification or Z-axis control. I
have since found a Tektronix RM503 oscilloscope. This was actually marketed by DEC as the VR01-A for use as a point plot display.
This new scope necessitated a modification to the
intensification circuits of the controller. The technicians scope (and
the HP scope I also have used) required a negative going pulse to
intensify the displayed point. The Tek scope requires a positive pulse
for intensification. A simple transistor inverter was added to support
the new scope.
One of the goals of using this point plot display was to get some version of a video game working.
After some help getting the EAE functional, I got a version of
The logic is almost identical to the VC8I.
The only deviations were to remove the light-pen and to utilize the Flip-Chips that I had available.
More to come.
Thanks to David Gesswein for providing the test code for the initial display pattern shown in the
He also suggested a few more programs to try.
I have found at least 2 versions of PDP-8 Spacewar to play with.
That will require some control box hacking.
I have DR8/E that should work.
SPACE.PA - This is pretty simple and intended for a LAB-8
SPCWR3.PA - This is much more complex than the previous.
It has compile time options for several different point-plot controls and assumes an EAE.
SPCWR3.PA - This is a version for the VC8I which I have not tested. It includes an update for a VC8E and a DK8EA.
CLOCK.PA - This is a program that displays the time
in digital format on the display. The clock rate is controlled by the
DK8EP Programmable Real-Time Clock Option. The characters are formed
from bitmapped fonts.