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Henryk Richter

A590 ROM, Bus Power

ROM update, no external PSU

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Amiga 500 formatting the SCSI ZIP drive, Kickstart 1.3

The Commodore A590 is quite an interesting harddrive. It comes along with actually two controllers. One of them is for cheap XT harddrives (mine came with one, non-working of course). The other controller is a classic SCSI controller that provides internal and external ports.

Furthermore, the A590 can host up to 2 MB DRAM (256k x 4 chips). Luckily for me, I was able to obtain 8 of these chips (in exchange for a genuine MC68000P8). This way, I could double the available RAM for this particular Amiga of mine.

The other two initial steps I was going to perform on the A590 were the replacement of the ROM by the latest version and the mod to a BUS-powered operation instead of using the bulky second power supply.

The first matter at hand however, was the problem of getting a working SCSI drive. Rummaging through my archive I found a good old Iomega ZIP drive, along with originally sealed disks. The old SCSI ZIP drive was recognized immediately as hard drive. What didn't work was the auto-detection of drive parameters in the HDtoolbox which came along with the A590 install disk. As it turned out, the inquiry response of thse ZIP drives offended more than just the HDtoolbox. My settings for the drive were: 792 cylinders, 4 heads and 62 sectors per track. The ZIP cartridge partitioned and formatted nicely with these settings. The correct settings I've seen elsewhere would be "cylinders=96, heads=64, sectors/track=32". I didn't try the latter settings yet, though.

ROM Upgrade

The initial ROM versions on the A590 were quite limited in terms of maximum harddisk size. Initial versions (6.x) were limited to 512 MB, later 1GB drives. Those sizes were well beyond anything available for purchase in those days - but nowadays people are used to bying terabytes of storage. Nevertheless, the 6.6 and 7.0 versions of the A2091/A590 SCSI controller natively supports 4 GB drives.

In conjunction with modern (or patched) file systems, drives bigger than 4 GB can be used when the last version of the ROMs is installed. A prerequisite for large drives is a newer Kickstart (3.1), though.

The files necessary for EPROM burning are floating around the internet (you might search for a2091-7.lha). The ROMs on the board have a data bus width of 8 Bit, so that one doesn't have to worry about byte swapping. One may choose any pin-compatible EPROM with 200ns (or better) access time. The best fit is the 28 pin DIP 27C64 EPROM with 8 kByte, directly matching the previously installed types. In fact, my device already carried two EPROMs. Other pin-compatible EPROMs would be the 27C128 or 27C256. In the latter two cases, I'd recommend to replicate the files twice or four times, respectively before burning (e.g. in OSX/Linux: cat a2091PDMA-07.U12 a2091PDMA-07.U12 >a2091PDMA-07_twice.U12). By that step one can avoid potential problems with the upper address lines of the socket with larger EPROMs.

In my case, the two 27C64 EPROMs (ST M264C64A) worked out of the box after burning the two aforementioned images.

BUS powered A590

While searching for information regarding the A590 drive, I stumbled over an apparently very popular mod among the A590 owners. Many people replace the failing SCSI drives by flash cards and as a consequence, dramatically reduce the power requirements of the A590. Without the loud and bulky (XT) or just loud (SCSI) hard drive, the power supply of the Amiga itself can provide enough juice in many cases. In my setup, the A590 PSU began to produce irritating noise when I removed the harddisk.

So the plan was to provide BUS power to the A590, following the approach outlined on Dave's page. One potential issue with Dave's approach (also mentioned by him) is that you should never connect the A590 PSU to the still existing power input after finishing the modification.

In essence, my choice was to grab the 5V and 12V power lines from the expansion connector and loop them back into the A590 power input port. This way, one actually "sees" that there is another power source and still has the ability to swap the BUS power with the external PSU input. In any case, the 5V line on the expansion bus can be found at Pin 6. The 12V line is Pin 10. These two pins are convenient, because there are no traces directly in the way. I glued the two cables to the board at convenient places. The wires then lead out of the casing through the screw-hole near the power input connector. To avoid damage to the cables, I covered them with heat-shrink.

I'd recommend to read Dave's page first before attempting the conversion.

What you need for my approach is a DIN connector. I only found a common 5-pin male audio DIN jack in my storage. As it is not needed to populate the ground and sense pins, I just dremeled away the inner 3 pins. The remaining two pins for +5V and +12V were bent a little to align with the power socket of the A590. Please refer to the images below where the 5V and 12V lines are connected in the socket.

A590 PCB overview with internal power wires attached
A590 PCB closeup depicting Expansion connector pin numbering
Location of 5V and 12V lines on the A590 power input socket
Finished BUS powered A590 with loopback cable
Modified 5-pin DIN plug, where the inner 3 pins are gone for good