The CoCo’s lesser-known screen color, revisited.

In a previous article I discussed the Radio Shack Color Computer’s lesser known text mode screen colors. Below you can see the normal, normal reversed, alternate and alternate reversed color modes:

During a recent discussion with CoCo FPGA programmer Roger Taylor, he mentioned something I was unaware of. He has spent years working on an FPGA recreation of the various Color Computers and casually mentioned a difference in the colors of the video display generator chip found in later model Color Computer 2s.

These machines, while cosmetically similar to previous CoCo 2 models, are identifiable by the case badge reading “Tandy” instead of “TRS-80”. If powered up, you can also tell the difference by the font. The Tandy Color Computer 2 machines had a round O instead of the square O:

The Tandy machines also have a zero with a slash through it, though that is not showing in either of those screen shots.

In addition to having “Tandy” on the outside, and round Os and slashed zeros on the inside, these model Color Computer 2s also had actual lowercase letters instead of inverted uppercase letters that represented lowercase:

You could get a Tandy CoCo 2 in to lowercase using these POKEs:

10 PRINT "Look, ma! Lowercase!"
20 POKE &H95AC,57:POKE &HFF22,PEEK(&HFF22) OR &H10
30 GOTO 30

The thing I did not realize was that the colors are a bit different between the original 6847 VDG chip used in the TRS-80 CoCo 1s and 2s and the 6847T1 VDG chip used in the later Tandy CoCo 2s. Roger was aware of this since he had been working on faithfully emulating the differences between the chips.

I discovered that the XRoar emulator already handled this. Consider this example, based on a suggestion by Cathe Cita in the Color Computer Facebook group:

10 CLS
20 PRINT STRING$(128,255);STRING$(128,255);
30 SCREEN 0,1
40 GOTO 40

The intent of this code is to clear the 32×16 text screen, then fill the top half with CHR$(255) — orange-ish blocks.

Code Note: For speed, I used “PRINT STRING$(num,char)”, though I could have also done something like “FOR I=0 TO 255:PRINT CHR$(255);:NEXT” but that would have been much slower.

It then switches in to the alternate color set with SCREEN 0,1. One a pre-Tandy CoCo with the original VDG chip, it looks like this:

TRS-80 (6847 VDG) CoCo CHR$(255) versus SCREEN 0,1 color

But on the Tandy model with the 6847T1 VDG chip, it looks like this:

Tandy (6847T1 VDG) CoCo CHR$(255) versus SCREEN 0,1 color

I do not have a real Tandy CoCo 2 here to verify the colors against this XRoar emulator example, but I am told they are accurate.

This means the T1 color set is more like this:

While I was aware of the lowercase and improved font of the 6847T1 VDG, these alternate colors were a surprise to me. Why did Motorola change it? Did they change it on purpose? I know the 6847 was used in several other computer (and at least one game) systems, but was this 6847T1 variant used elsewhere?

Let me know in the comments.

A colorful finale

And here’s all the variations I know of, so far, 6847 and 6847T1, in one gallery:

Until next time…