Date   : Fri, 12 Nov 2010 18:44:35 -0000
From   : jumbos.bazzar@... (Mark Haysman)
Subject: ROMs

> ----- Original Message ----- 
> From: Ben Collier

> I was wondering if anyone could tell me which sorts of chips will work as 
> ROMs in the BBC ROM slots.

Hi.

Assuming a standard Model B with the jumpers set for a 27128 OS in the left 
most socket, you can use 2764 and 27128s in the slots, and each rom can 
contain 1 image. You can use "some" brands of 27256, but as pin 1 is 
floating, some EPROMs will give spurious data as that address line is not 
connected. Same with 27512, but whilst pin 1 is floating, pin 27 is 
connected to Vcc, so for either of these, if you do find a brand that works 
reliably, you'll still only be able to use one image.

That is, unless you modify the main board, as per 
http://www.retroclinic.com/acorn/swr/swr.htm (Ignoring the SWR part). That 
will then allow you to use a 27512, and for it to contain 4 separate ROM 
images. You can also do hacks such as fitting a switch to hanging pins etc.

For a B+, the sockets can be jumpered to take either a single image in a 
27128 or 2 images in a 27256. Same for 2 of the 3 free sockets on a Master, 
they are 27256 compatible, but share space with SWR via a jumper. The socket 
for ROM 8 can only take 1 image, although because pin 1 is now tied, you can 
reliably use a 27256 or 27512, but again for only a single image.

> Are there commercially available chips of one sort or another which will 
> fit and are easily programmable?

You can still buy 27256 and 27512 new from major suppliers, or any of them 
as NOS or erased off eBay, they're pretty common. Cheapo programmers are 
also available for the PC, or you can get Beeb ones.

Mark.