[Coco] MC6847 External ROM boards finally ready...

[Craig Michael]

w00t!! Money sent! Can't wait to get my paws on this! :-)
Mike 


     On Sunday, August 23, 2015 1:47 PM, Zippster <zippster278 at gmail.com> wrote:
   

 Well, they are finally ready!

The board is specifically designed to fit American CoCo2 motherboards, but should work in
any 6847 socket, as long as you have physical clearance to plug it in.

It supports either a 27C256 or 27C128 or other ROM/EPROM with the same pinout.
At the end of the email is information on usage of the 7 position DIP-switch.

There are 12 character sets on the ROM that comes with it, several are variants of the same
typeface tweaked for better appearance in some situations.

The first 2, supplied by Al Hartman, are the original Lowerkit character sets by Dennis.
The 3rd, also from Al, he believes, is a set made by Marty Goodman from the set on a IBM PC graphics
card way back when.  The others I put together based on some fonts found out on the internet.

Here are some pics of the included character sets…
<https://drive.google.com/open?id=0B1sgl615FlGnfl9ZMWhqeGstWlRIYnRHRVJSaHUyeVRaa29ScVdmWmZUWEFlakM5WW1GUHc <https://drive.google.com/open?id=0B1sgl615FlGnfl9ZMWhqeGstWlRIYnRHRVJSaHUyeVRaa29ScVdmWmZUWEFlakM5WW1GUHc>>

I put together an Excel sheet that makes it easy to create sets.  On the right side of the sheet is the 
hex code for the set, which can be copied and pasted into a hex editor to make a .bin file.
Here are the sheets I used to make the ROM.
<https://drive.google.com/open?id=0B1sgl615FlGnfmdlMnoxVWlmV1RxTjhSb09fTGhsblpCWHAwbEdUY2E2X0ZVQ3BvTW1DS2c <https://drive.google.com/open?id=0B1sgl615FlGnfmdlMnoxVWlmV1RxTjhSb09fTGhsblpCWHAwbEdUY2E2X0ZVQ3BvTW1DS2c>>

Hopefully we can make some more cool character sets for the 6847.  If you don’t have an eprom burner
already, this might be a good excuse to get one.  :)

-------------------------------------------------------------------------------------------------------------------------------------------

I built 28 of these all together, 17 are spoken for, and I’m keeping 3, so there are 8 still available for
anyone interested in experimenting with some alternate character sets for the MC6847.

The boards are $32 ea + shipping.  Shipping is $5.95 in the US, $15.00 internationally.
My Paypal is zippster278 at gmail.com <mailto:zippster278 at gmail.com>

Also, if anyone needs an MC6847 because they put a socket in or something, I have a number of these available ($6).

Ok, I think that’s it.  Thanks guys!

- Ed





MC6847 External Character ROM - REV 03

DIP SWITCH SETTINGS

1 - Enable External ROM
2 - Inverse mode
3 - All Caps mode

4…7 - Bank selection 0…15 in binary


1) - Enable External ROM
Selects between MC6847’s internal ROM, and our external ROM.

2) - Inverse mode
Inverts the normal use of the INV line.  What was black on green is now green on
black and vice versa.

3) - All Caps mode
To accommodate certain programs that are expecting inverted uppercase when
the computer calls for a lowercase character from the internal ROM, this mode 
does the following.

Decodes for any VDG calls for a lowercase character in external ROM (610h - 7A0h),
and offsets the address lines to the ROM chip for uppercase.  Also triggers inverse mode
for that character.

The results will be like using the internal MC6847 ROM, but with the external
character set.  Inverted uppercase in place of true lowercase characters.

4…7 - Bank Selection
Selects one of 16 character banks to use in binary


        Bank 0    Bank 1    Bank 2    Bank 3

        0 0 0 0    0 0 0 1    0 0 1 0    0 0 1 1
        - - - -    - - - -    - - - -    - - - -
Switch  4 5 6 7    4 5 6 7    4 5 6 7    4 5 6 7    etc….

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