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Commodore PET Projects



Steve Gray's Multi-ROM Board


When I started to supply petSD+ (an SD card disk drive replacement for the PET), it quickly became apparent that, to get the most out of it, the user should really have a DOS Wedge installed. Nils Eilers has written a DOS Wedge that can be loaded as a TSR program, but the most convenient method of using a Wedge is when it is incorporated into the PET Editor ROM.

Steve Gray has done a lot of work to create enhanced Editor ROMs with a host of extra features, including Nils' DOS Wedge. Using Steve's tools, I made a few custom Editor ROMs available to petSD+ users as drop in replacements for their existing Editor ROM. These worked fine, but were an "all or nothing" replacement, if for some reason, the user wanted to temporarily revert to the original ROM, they needed to physically swap out the chip. A better solution is to allow the user to select between available ROM images using DIP switches. - That's where Steve's Multi-ROM board comes in. Originally designed as Editor ROM replacements, Steve now has boards that can replace most ROMs in a PET/CBM, including the character ROMs. (There are some limitations with the SuperPET, but as most users don't have one, I won't go into detail here, see Steve's page for more info.)

[As well as Steve's Multi-ROM boards, Jim Brain at Retro Innovations also has adapters that can convert legacy 23xx and 6540 pin-outs to allow the use of 2764-27512 ROMs.]

Steve has also developed a tool (his Multi-EditROM Builder, MERB) that creates ROM sets for use with his Multi-Editor and Multi-ROM boards and has spent a lot of time generating libraries of ROMs that can be downloaded from his space on Github.

Steve has kindly allowed me to make these Multi-ROM boards available to petSD+ users, if you are considering using a replacement Editor ROM, or indeed, want the flexibility to select different ROM images in a particular slot, you should seriously consider adding one (or more) of these great boards to your system. As well as Editor ROM selection, other uses would be to select between different copy protection ROMs you might have, such as PaperClip, WordPro, VisiCalc, etc.

Another advantage of these boards is that you don't need have the hassle of trying to find and/or program obsolete and hard to find legacy EPROMs. 

PET Option ROM Memory Mapping

PET/CBM computers were shipped with two unpopulated ROM sockets that could be used for various third party options such as Application program code or copy protection ROMs for application programs supplied on disk. The photo at the top of the page shows the computer board from my CBM 8032/8096, the two unpopulated sockets are the Option ROM sockets, in this case, they are labeled UD11 and UD12. In other models, they board positions are different, but the option ROMs are mapped into the same address spaces as shown in the table below :

PET/CBM Model  PCB P/N  Option 1 
Option 2 
2001    ??  ?? 
2001N/3000/4000    UD3  UD4 
8032 8032039  UD12  UD11 
Universal 8032081  UD12  UD11 
Universal 8032087  UD12  UD11 
8296    UE10  UE9 

Particular option ROMs were intended to be installed in a particular slot so that they executed in the intended address range, so the number and combination of ROMs that could be installed was pretty limited. With the Multi-ROM board, each socket can have up to 16 ROMs available for use and selectable using the DIP switch.



(A number of the images on this page are from Steve's Multi-ROM page)

The KiCad model of Steve's Multi-ROM adapter PCB

The adapter uses readily available 27512 EEROMs, these are much easier to source than the legacy 2332/2532 ROMs used in PET/CBM computers.
The 27512 can hold up to 16 x 4k ROM images, normally selectable using a 4 position DIP switch mounted on the end of the PCB.

It is possible to leave off the DIP switch, and instead, mount up to 4 toggle switches on the PET case for ease of access to the switches. 

I don't intend to offer the remote switch mounting option as I find that it's easy enough to lift the hood of my PET and change the bit switches if I need to. (I don't have the two case hood screws secured in my case unless I am moving the PET around.)

If you really wanted to do it this way, I could be persuaded to get the necessary parts together for you but the external switches etc. would probably cost as much as the complete PCB.
When assembled, the completed board looks like this 
If you want to configure the ROM image to your own specification, you can use Steve's Multi-EditROM Builder, available from his page on Github

The tools allows you to select up to 16x4k ROM images, each of which can be put into any of the available 16 slots. Full instructions are available on the webpage.
For most users though, the easiest and best option is just to choose one of the ROM sets that Steve has pre-prepared and made available on his Github page

For example, Edit-ROM Set #01 for 80 Column PETs 
Or Option "A" ROM Set #01 which includes items such as the copy protection ROM for the PaperClip Word Processor and various WordPro ROMs
The Option ROM sockets fitted to many PET/CBMs are either poor quality and/or have unusual frames that have square holes for the ROM pins.

If the Multi-ROM board cannot be plugged directly into one of these sockets, either due to damage to the socket or poor contact between the socket contacts and the straight pins of the board, then  attaching a standard socket to the Multi-ROM board before installation should alleviate the problem.
The Multi-ROM board populated with a 27512 EPROM and installed in position UD11 on my CBM 8032 board. With the DIP switches set to "0000", the Execudesk Word-Processor (PaperClip) has found what it thinks is the PaperClip protection ROM and loads without problem.

The angle of the photo suggests that there isn't room to fit a second Multi-ROM board in position UD12 but that is not the case  . . . .
A second Multi-ROM board installed in UD12, loaded with the Option "9" image.

With the DIP switches set to "0000", my Version 1.75A of VisiCalc which checks for the presence of a software protection ROM at address #9000 loads without problem, confirming that the Multi-ROM board is again performing as required.
With Steve's permission, I can supply the Multi-ROM board, loaded with the ROM set of your choice, for 11.00 (a small portion of which will be passed on to Steve). Most people will get the biggest benefit from using the board to add a Wedge enabled Edit ROM to their PET for use with petSD+, but some users may find them useful for adding alternative Character ROMs or Application ROMs to their system.

The ideal time to purchase one of these boards would be at the same time as you are adding petSD+, but for other orders, please send me an e-mail 

I have a very limited quantity of the boards but will order more should they be required.

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