[Coco] MC6847 External ROM REV02

[Zippster]

Camillus,

As Joe said, the MC6847 supports an option for an external character set ROM in addition to
the built in (uppercase only) character set.  More information on this and how the code is formatted
can be found in the MC6847 data sheet.

An external ROM lets you implement a “full” 128 character set, so that you can have real lowercase characters.

Early boards existed for this, like the lowerkit boards.  These used a 2k EPROM with one character set.

I’ve designed this one to use either 27C128 or 27C256 EPROMS, allowing for 8 or 16 character sets,
selectable by dip switch.  There are a couple of spare switches on the dip, and all ROM addressing is
run through the CPLD, allowing for other features, such as redirecting addressing for an inverted uppercase
character instead of a lowercase one on some programs that expect this, maybe other things as well.

I’ve also put together an excel spreadsheet that allows you to generate the code for character sets
easily, building the characters in a graphical fashion, for burning to ROM.

- Ed


> On Apr 17, 2015, at 8:01 PM, Joe Grubbs <jsgrubbs at hotmail.com> wrote:
> 
> The ROM is to contain a new character set which you can define yourself. The 6847 offers the facility for an external character generator, and this PCB is just that :)
> 
> 
> 
>> Date: Fri, 17 Apr 2015 19:41:02 -0500
>> From: camillus.b.58 at gmail.com
>> To: coco at maltedmedia.com
>> Subject: Re: [Coco] MC6847 External ROM REV02
>> 
>> External rom, what is the idea to put in it?
>> 
>> Can cocoprograms ( bin) be put in there and run from this board?
>> 
>> What is the price?
>> cb
>> On 4/17/2015 7:21:33 PM, Zippster <zippster278 at gmail.com> wrote:
>> Ok, looks like we have 5 already. :)
>> 
>> - Ed
>> 
>> 
>>> On Apr 17, 2015, at 11:46 AM, Luis Antoniosi (CoCoDemus) wrote:
>>> 
>>> Oh yes, this one I'm interested :)
>>> 
>>> 
>>> Luis Felipe Antoniosi
>>> 
>>> 
>>> 
>>> On Fri, Apr 17, 2015 at 11:22 AM, Craig Michael
>>> wrote:
>>> 
>>>> Oh coolz!! Put me down for one! :-)
>>>> 
>>>> 
>>>> 
>>>> On Thursday, April 16, 2015 8:55 PM, Zippster
>>>> wrote:
>>>> 
>>>> 
>>>> Just ordered boards from OSH Park for REV02 of the MC6847 External ROM.
>>>> 
>>>> 
>>>> https://plus.google.com/118352500538051706779/posts/d3HV2p5iqn2>>
>>>> 
>>>> 
>>>>>> 
>>>> 
>>>> If all is well when I get them, I’ll do a small run of these for people
>>>> who are interested. :)
>>>> 
>>>> - Ed
>>>> 
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