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Representing IBM 5153 color output more accurately


Veteran Member
Jul 21, 2011
Refer to [here]. The contents of the ROM within the IBM 5154 make any colour changes.

As for 200-line mode (a.k.a. CGA, a.k.a. mode 1), IBM have published the RGBI-to-rgbRGB mapping done by the ROM.
That mapping is shown in a table on page 4 (PDF page 8 ) of IBM's technical document for the IBM 5154 (at [here]).
In the 'Brown' row of that table, the reduction of green to achieve brown results from 'Gg' being '01' (G=0,g=1).
It would be '10' for yellow.

Regarding EGA, and the 5154 in particular... is it possible to establish the actual relationship between the primary (MSB) and secondary (LSB) signals, in terms of levels? (Modulo any funny business with contrast controls.)

I've been assuming that 'G' is twice as bright as 'g', so that 'Gg' in turn is three times as bright. That's how the 6-bit RGB model is usually interpreted. But thinking about it, there's no reason to strictly mandate that exact relationship, and I couldn't find anything in IBM's documentation for the EGA and ECD to clarify this.

Sams Computerfacts for the 5154 VDU could probably help, since it clearly draws up the signal path through the Red/Green/Blue amps to the CRT board. But admittedly, I'm less than competent in interpreting these schematics.


New Member
Nov 5, 2021
What it is; a Motorola CPU that has its UV Eprom built into it, so I cannot read out the eprom file to make a duplicate. Though there was a fellow who offered to help, by using the original programmer circuit and doing a type of error check, it is too risky to send overseas, because if anything happens to the file in the Rom, the entire monitor is toast.

This sounds like a Motorola 6801 or similar. It is not too hard to build the appropriate circuit and program to read out the internal mask ROM; for a 6801 page 2-8 of the manual is where you want to start:

The significance of the first feature [of Mode 0] is that an external program can be used to obtain control of the MCU from Reset and all other references to the interrupt vector area (such as MPU reads) will access internal ROM. This characteristic provides a method for reading the entire ROM including all of the interrupt vectors from a program which resides in external memory.