For output the MCM/70 used a Burroughs Self-Scan Model C4047 plasma display. These Self-Scan displays were an important stepping-stone technology between teletype terminals of the 1960s and the widespread use of CRTs from the mid-1970s on.

The display was arranged as a 222 column by 7 row dot matrix.  An MCM/70 would use these dots to render 32 characters of 5x7 dots with two blank columns between each.  Now a single line of text with 32 characters doesn't sound like much, but considering that the Altair 8800 released that same year just had front panel blinkenlights out of the box, it was quite a step up.

Here's the problem. These plasma displays are no longer manufactured. You can occasionally find one on eBay but they are pretty expensive. Even if I could source one, to tell you the truth the high voltages required for the plasma elements (250V) scare me a bit (even though I know the amperage is low).

So I spent a lot of time looking for a suitable replacement and think I found one, the Broadcom HCMS-2972.

Packaged as 8 5x7 dot matrix arrays these modules operate at a nice safe 3.3V and can be cascaded together side by side to create the 32 x 1 character display desired here. Furthermore the viewing area for the 32 characters measures 171.7 mm x 4.57 mm while the Self Scan viewing area is 181.61 mm x 4.98 mm.  So very very close. 

There is one small drawback to using the HCMS-2972s. The Self-Scan displays form a full 222 x 7 dot matrix. When displaying characters however two columns between each character are turned "off". Four HCMS-2872s side by side are only 160 x 7 dots where there are physical gaps between the characters that happen to be about 2 columns wide. For normal use there will be no difference in the displays since the MCM/70 always keeps characters on a even 7 column boundaries. However when an MCM/70 is executing instructions that do not involve displaying outputs, it uses display memory to store temporary values.  Since the hardware is constantly updating the display from this memory, random looking patterns of dots will be displayed. In my implementation these "thinking" patterns of dots will have blank columns between where the characters are. I think I can live with this.

I received a HCMS-2972 earlier this week and setup quick test using an Arduino Nano I had kicking around. Here is what it looks like fired up.

I have 3 more HCMS-2972s on order now.