DEC VT100 character generator

Jonathan Gevaryahu jzg22 at
Wed Nov 25 04:50:25 CST 2009

On 25/11/2009 02:04, Jonathan Gevaryahu wrote:

[ stuff which was below a signature line so it didn't come out ]

The ROM image came from from someone I've got data from before, who 
fixes things, and says he dumped it from one he desoldered from a 
working VT.  I have no reason to doubt him.

The idea that DEC made a practice of arranging the checksum to come to 
zero or end "00" is news to me.  I'm fairly sure that's not the case. 
There must be many exceptions, if only because there's often not a spare 
location to store the necessary constant to adjust the checksum.

Having worked with DEC stuff when it was current, even though it was a 
long time ago, I'm quite familiar with the ROMS, including in particular 
Q-Bus devices and VT1xx devices, because I used to repair them, to 
component level, for a living.  In fact that's why I started collecting 
the images.

Using different polarity chip selects on masked ROMs is common practice. 
  Many manufacturers did that, and it's not hard to get round.  If your 
programmer can't deal with it, just bend the pin carefully and use a 
test lead to tie it high or low.

Pete						Peter Turnbull
						Network Manager
						University of York

Apologies for doubting you then, I'm just surprised the rom matches. 
There actually IS spare space in the character generator rom for the 
vt100 since bytes xxF xx0 xx1 xx2 xx3 xx4 xx5 xx6 xx7 xx8 are used for 
rows 0 thru 9 of a given character xx(yes byte xxF comes first, I don't 
know why DEC did that but it says so in the vt100 tech manual), but 
bytes xx9 xxA xxB xxC xxD xxE are unused and can have any data stuffed 
into them.

I'll mark the rom as good in MESS; who dumped that rom by the way? I 
want to give proper credit (unless they wish to remain anonymous).

Jonathan Gevaryahu

More information about the cctech mailing list