PET ROMs

Introduction

The oldest machines in the Commodore PET/CBM range are now getting on for 40 years old, as might be expected for computers of this vintage, many of the original components are obsolete and difficult to source, this includes the ROMs. Even worse, when replacement (EP)ROMs can be sourced, many of today's programmers are not capable of reading and writing these legacy components.

In most circumstances, this is not a problem unless one of the original ROMs fail, but if you want to upgrade a machine, say from BASIC 2 to BASIC 4, then this situation does become an issue. In my case, the original ROMs in my CBM 8096 were working perfectly, but I have replaced the Editor ROM with a copy of Steve Gray's Editor ROM which incorporates Nils Eilers DOS wedge. When I was testing petSD+ and struggling with the BASIC 4.0 syntax to control it, Nils had extoled the virtues of a DOS wedge and kindly created the new Editor ROM for me.

Having the DOS wedge integrated into the OS is really great and is something that I wanted to be able to offer to petSD+ users, this meant that I had to learn a little about the different potential ROM configurations that I would need to be able to generate and the tools that I would need to do so.

Nils advised me of the comprehensive list of the ROMs from Commodore computers compiled by Martin Hoffmann-Vetter and posted on his website and noted that Commodore used 6540, 2316, 2332 ROMs and 2532 EPROMs. With the exception of 6540 ROMs, all of them can be replaced by a 2532 EPROM. Of the machines that did use a 6540, models with BASIC 1 are of no interest as they do not support IEEE-488 disks and the remainder are pretty uncommon, so at this stage, I shall just be focusing on being able to create Editor ROM images that can be written to a 2532 EPROM.

The 2532 EPROM is obsolete but they are available from a few sellers on eBay, the majority of them are advertised as "refurbished" - which I guess basically means "pulls" that may or may not have been UV Erased and had their legs bent back into shape. From what I have seen so far, it seem that the vast majority if 2532s were made by Texas Instruments, the only other manufacturer that I have come across is Motorola (MCM2532) but Nils mentioned that Hitachi also made a version they called the HN462532. The TMS2532 was produced in two variants, the 2532 and the 2532A, the difference being the programming voltages used, 25V in the case of the 2532 and 21V for the 2532A.

Programmers

At first sight, it appears that many of the cheap EPROM programmers sold on eBay, including my TOP853, do not support the 2532 and I thought that I was going to have to buy a new programmer. However, a search for "program 2532 eprom" flagged up this post on 99er.net that describes how to make a simple adapter to allow a 2532 to be programmed in a device that supports 2732 EPROMs, which my TOP853 does.

The pin-outs of the 2532 (on the left) and the 2732 (on the right) show that the only differences are in pins 18, 20 and 21. Function 2532 2732 Vpp 21 20 PD/E 20 18 A11 18 21 The adapter just cross wires the pins on the 2532 to the positions used on a 2732
This is one that I knocked up to prove that this adapter would work. I used a turned pin socket for plugging into the programmer (I only had a 32 pin to hand) and a cheap spring socket for the EPROM, with patch wires soldered between the relevant pins.
I did not have any spare 2532s at this point, but did have a redundant copy of the Editor ROM that Nils had given me. I installed this in the adapter and fired up the TopWin software to see what 2732 options were available. I found that TopWin supports a number of 2732s, including an option for a generic 2732 with programming voltages of 12.7V, 21V and 25V and was able to read the 2532 without error.
To confirm that I could write to 2532 EPROMs using this setup, I bought some test pieces of eBay, two each of used TMS2532 (25V) and TMS2532A (21V). Although the advert advised that they may not have been erased, I found that they were blank If it had been necessary to erase them, I do have a UV Eraser.
When I tried to write the first EPROM, the write failed about half way through and the failure was repeatable. Since it was not failing at the first byte and having seen similar problems when writing some EEROMs, I suspected that it might just be a timing problem. I experimented with the  "Delay" parameter in the TopWin "Config" tab and found that 5ms worked reliably. I need to do a few more to be 100% certain, but it seems that I will able to create Editor ROMs with this setup.
Having proven that the adapter works, I decided that I would like something a little more robust. I designed a simple adapter board for my TOP853 programmer and had some PCBs made up. Here I have assembled the first one using a turned pin socket that I had to hand but I have ordered a 12-pin ZIF socket for the final version. I have some spare PCBs if anyone is interested in buying one. Send me an e-mail
Prices include UK P&P, but exclude any payment fees, such as Paypal charges. Prices for the socket options include fitting the socket and pin headers for connecting to your programmer.	Bare PCB £5.00 With spring socket £7.00 With turned pin socket £8.00

Making Steve Gray's Editor ROM

Steve has developed a method of producing customised Editor ROMs for PET/CBM computers, for full details, see the Project web page on his website. Steve's page provides a comprehensive description of the features and options that can be included, including portions added to support his Colour PET project.

To create a custom Editor ROM for a given system, the required options are selected by setting appropriate values in Assembler Directives in the EDIT.ASM source file and compiling the package using Marco Baye's ACME Assembler to generate the binary file to be programmed into EPROM. (Many of the options are either not relevant, e.g., the Colour PET options, or outside the scope of my desire to make Editor ROMs with the Wedge and a few other functions, to support petSD+.)

The assembler is hosted on SourceForge on the ACME Cross-Assembler page, a Windows binary for Version 0.95.6 is available from emu64.de (a Commodore 64 Emulator). There is no install program as such, you just need to extract the files to a directory of your choice, preserving the directory structure within the archive.

The source files for Steve's Edit ROM are hosted on GitHub at sjgray/cbm-edit-rom, again, download the archive to a directory of your choice, preserving the directory structure. Compiling the binary is done by running the make.bat batch file, which just runs the command "acme edit.asm". The ACME assembler should either be in the search path, or, for simplicity, in the same directory as the Editor ROM files.

Nils had given me a copy of the EDIT.ASM file used to generate the Editor ROM that had been fitted in my 8096. To confirm that I was able to create ROMs from source files myself, I compiled this file and wrote the resulting binary to EPROM and installed it in my CBM8096.

Result! - It seems to work flawlessly

Making More . . . . .

A sample EDIT.ASM file is available here, it is the file used to build the Editor ROM for my CBM8096. As I mentioned earlier, it contains quite a lot of configuration entries that will not apply to the majority of people. The table below contains a subset of the configuration items that should allow me to build an Editor ROMs for other systems.

The options greyed out as "Not currently offered by me" are either because the function is not fully implemented in the Editor code, or I don't fully understand the implications of applying the option. If someone really wants one of these options configured, I will do so, but can't take any responsibility for the impact on the usability of the target machine.