Date   : Fri, 10 Dec 2004 19:01:28 -0800
From   : Angus Duggan <angus.duggan@...>
Subject: Eprom Programmer 

Mick Champion writes:
>To the BBC list, 
> 
>Sprow wrote : 
>Sprow> 27128 - NMOS, 21V for Vpp 
>Sprow> 27128A - NMOS, 12.5V for Vpp 
>Sprow> 27C128 - CMOS, far too weedy to drive the bus 
> 
>I have had 3 working 27C128s installed in my model B for two years now. All 
>were programmed using a programmer that I built using Angus's superb design, 
>so it is possible. The 20k resistor on the TL497A had to be changed to 15K to 
>reduce the programming voltage to 13v. The resulting heat reduction inside the
>BBC is great as C128s hardly get warm. 
> 
>There was a also a mistake in the 1985 version of EPROG.PDF. The correct 
>wiring, is for both 5Ds on the two 74LS374's to be strapped to PB4, and both 
>6Ds to PB5. On my PDF version, the right hand 74LS374, shows 'incorrectly' 
>that 5D goes to PB5 and 6D to PB4. The PDF may have been updated by now, 
>ignore if it has. 

In an uncharacteristic burst of energy, I have updated the original circuit
diagram and the PDF file, and put the new ZIP file on my website. I made the
PB5/PB4 change, and added a 60k resistor in parallel with the 20k one with a
switch to change between 21v and 12v programming. (According to my
calculations, that should do the trick.) The new version is at:

  ftp://ftp.knackered.org/pub/angus/BBC/eprog.zip

Mick, you had mentioned putting a 16v 47uF capacitor between pin 7 and ground
of the TL497A. Did you implement that?

There is another change I had mentioned, but haven't updated the circuit for;
I'll wait until I implement it successfully until I update the circuit (maybe
another 10 years...). The other change is a pull up/pull-down switch and a
74LS00 to be able to program the high and low addresses of 27256 EPROMs
separately. With a tiny bit of circuitry to grab other address select lines,
these could be used in a standard Beeb to double the number of usable ROMs.

a.