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VT220 != VT220

Roland Huisman

Roland Huisman

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Mar 24, 2011
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I've been fixing a few VT220 terminals lately. There seem to be an old and a new model VT220. The old model (on the right hand side) is from 1984 and the online 1983 documentation matches the hardware. The terminal on the left hand side is made in 1987. The hardware is completely different. And I can't find any documentation on that model.


verschil 1.jpgverschil 2.jpgverschil 3.jpgverschil 4.jpg

The new and old style CPU boards can be exchanged between the terminals.

VT220 CPU 1987.jpgVT220 CPU 1984.jpg

I wonder, does anyone have the documentation of the later model VT220?

And does any one know how you can check the fosfor color in the serial number?

Regards, Roland
 
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That's actually really interesting. What internal event manifested for them to go through the throes involved for such a dramatic change. I wonder, noting that large, new shield, if it had to do with some new emissions requirements, so much as to require a brand new board.
 
My guess is that it was cost that prompted the change. Far fewer packages on the PCB. By 1987, DEC was seeing a lot of competition in the VT220 area. Heck, in 1983, we elected to use C. Itoh terminals on our VAX. Bigger screens and less money.
 
The managers at a large company I worked for elected to use C. Itoh terminals instead of DEC terminals on our VAX systems. Saved a bunch of money. The managers all patted themselves on the back for such a big win. Until the horizontal output transistor in each and every one of the C.Itoh terminals started failing. Hundreds of them all at more or less the same time. The users were not pleased. The floor of the IT offices were littered with dead terminals and C. Itoh techs soldering new transistors in for all they were worth. It was all covered by warranty, but that didn't really make up for the inconvenience. The system programmers were fine - we were DEC bigots and had insisted on getting real DEC terminals when we ordered the VAXes..the company ordered DEC terminals from then on.
 
Comparing the two boards it looks like they compacted a ton of discrete glue from the earlier board into that one flat-pack, along with the CRT controller. That was basically the story of the mid-1980's for you. It's a lot like the difference between, say, an original Tandy 1000 and the "Tandy 1000A"; the two machines are practically identical in function but the latter machine has the 6845 CRT controller and a bunch of supporting glue compacted into one proprietary ASIC, and a bunch of the CPU clock generation and arbitration goo (like the 8284) stuffed into another. Even if you couldn't make the PCB board physically smaller the cost savings of having to pick/place/solder fewer parts and resulting reliability improvements paid off, assuming you were assembling enough units to eat the initial cost of baking your custom chip.

(Which was getting more accessible at the time because companies were starting to license out the "formulas" for common support chips for inclusion in ASICs like this to third parties.)
 
Because of the ASIC, I suspect that the later VT220s are less repairable.
There were a lot of VT220 clones. I remember working on one marketed by TAB. Even Beehive had one. I still have the manual for a Televideo 922 (no longer have the terminal).
 
If someone has an LK201 with a bad membrane keyswitch assembly, I'd like to dump the 8051 on the encoder board.
The only LK201 that I have is a later version with a MC68HC705C4 microcontroller on it.
 
Reading Intel 8051 is usually no problem, they have no read protection. I used a GALEP5 programmer for this.
Attached is the ROM code read from the LK201AA Keyboard Encoder 8051 DEC Part # 23-004M2-00.
 

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