Pimping a TRS-80 Model 4 with a FreHD hard disk emulator


I love all-in-one computers. I especially love my TRS-80 Model 4. Not only is it supremely retro-looking but it also runs most of that TRS-80 Model I software I cut teeth on back in the day.

The TRS-80 Models III and 4 are natively floppy-driven meaning a certain amount of floppy shuffling is required to use them. However the machines do have the ability to use an external hard drive given the right operating system. These old hard drives are very rare and those that do exist are likely to be on their last legs.

When I heard about the FreHD hard disk emulator project I was intrigued. I'd never used a hard drive on a TRS-80 Model 4 before but felt it would be handy to have all my software in the one place. I decided to order a kit and give my Model 4 a storage upgrade!

Building the kit

Friend and Model III owner Philip Avery and I obtained two "B" kits from TRS-80 aficionado Ian Mavric. It arrived as a bare board with some of the harder components like the SD card holder and a few surface mount things already pre-soldered. A trip to the local electronics store and Philip and I soon had all the necessary components. Philip helped me construct a hard drive cable, something that wasn't in the kit.

....And then the project stalled for about 6 months. Other things just seem to get in the way and the yet-to-be assembled FreHD sat in a box quietly waiting for my attention.

About a month ago, I prioritised the project and assembled the kit (Image 1). By this time the ROM had been updated, Luckily a FreHD early-adopter/alpha-tester hardware enthusiast from Auckland, Andrew Quinn, was good enough to reprogram the chip as well as testing my newly-minted FreHD.

The naked FreHD

Image 1. The naked FreHD

After receiving the FreHD back from Andrew along with some disk images for the SD card, I was ready to mount the unit in its own case.

Housing the FreHD

Much as I regard bare circuits as works of art and enjoy gazing upon them, I wanted to house my FreHD in a case with a 5 volt supply. Not only would it look cleaner, but it would prevent desk clutter. Luckily I had just the thing. Earlier I've housed my HxC floppy drive emulator in an-ex SCSI tape cartridge case. There was enough room in the case and the front opening to mount the card above the HxC unit. I secured the front of the card with Velcro and the back with a couple of sticky mounting feet.

FreHD sitting nicely on an HxC

Image 2. FreHD mounted neatly above my HxC floppy emulator

It was then just a matter of splicing in the 5 volt line to the FreHD and adding the hard disk cable out the back. If I was being fussy I would have also tried to put a switch out the back to toggle power between the FreHD and the HxC but either one is fine at idle while the other is being used.

I now had a duel purpose device which could emulate:


Dual HxC emulator and FreHD

Image 3. Two for one. A FreHD/HxC floppy disk emulator combo

Why stick with plain old FreHD when you can have Autoboot FreHD!

The conventional way to use FreHD is to plug it in, prepare a hard disk image (or hard disk images) of your favourite Model III/4 operating system(s), partition it using the handy tools and instructions provided, and load it up with software. The hard drive emulator still needs to be booted from a floppy first though, just as a real hard drive would need to be.

The FreHD team had gone one step further than this. They had developed code which allowed a user to select and boot a selection of operating systems from a menu. No floppies required! This, I wanted!

To enable FreHD autoboot on a Model 4, one of the ROMS needs to be replaced with an EPROM containing modified code. A binary file is also needed on the FreHD SD card. As Model 4 ROMS differ depending on variant, it was important to identify the exact ROM set on my machine. Ian Mavric sent me through the ROM ID software seen in image 4. Once the ROM set was identified, Ian then sent me a BIN file to burn to an EPROM (Image 5).

What ROM does my Model 4 have?

Image 4: Identifying my M4 ROM

Replacing existing ROM with FreHD autoboot

Image 5. Replacing existing ROM with FreHD autoboot

After this I obtained some pre-configured autoboot hard drive images off Andrew Quinn and I was away. I now had Model 4 LS-DOS and CP/M, and Model III LDOS and NewDOS 2.5 loaded on FreHD hence available to my Model 4.

The next step was to load up all these images with appropriate software. No worries. Copying from my existing disks was a breeze (Image 6).

A utility program on FreHD allows users to load files directly from the FAT32 environment on the SD card to an HD image. However, I figured out how to configure the DOS programs to use the HxC floppy drive emulator as two external disk drives. As they were in the case with the FreHD it seemed a shame not to utilise them somehow. This means in the future if I find desirable sofware in the form of a disk image, I can simply mount these on the HxC floppy disk emulator and copy the programs to FreHD under an appropriate M4/MIII DOS. Sweet.

FreHD working hard

Image 6: The FreHD getting a software load

Last words

I'm very pleased with my autoboot FreHD configuration. I now have a self-booting Model 4, with four operating systems to choose from housed on an emulated hard disk, and access to two emulated floppies if needed. Disk access is fast and convenient. It's given the machine a brand new feel. I also like the way you CAN revert the machine to booting off a floppy (and ignoring FreHD) just by holding the space bar on boot.

It's not perfect yet. For example, on my unit changing operating systems often requires a double reboot. However, these minor foibles may be sorted out with software upgrades as time progresses and the community gets more familiar with autoboot FreHD. Also it would be nice to get some more operating systems in there too, such as MultiDos and DOSPlus. Such additions would require hard disk drivers though, which may not exist yet?

The process wasn't quite as easy as my writeup would suggest. I wasn't very familiar with LS-DOS or LDOS and had to learn the fundamentals of these fairly complex 8-bit operating systems to configure them properly. Also, autoboot FreHD (and FreHD itself) assumes a reasonably high level of TRS-80 III/4 literacy. Instructions (as yet) are likely to baffle newbies who might be new to the scene. I had an intermediate knowledge of the DOSes and Model 4 when I started but no I had no familiarity at all with hard disk configurations. It was a learning curve for me, and Ian and Andrew were a great help during this process. Thanks guys.

Happy little FreHD chappy

Image 8. Happiness is a snuggled up FreHD/HxC combo

Further FreHD development is ongoing. A the time of writing the FreHD community have got the card working on the Model 1, Dick Smith System 80 and Kaypro machines. If you're interested in your own FreHD (and want to retire those floppies) check out the following links:


17th May, 2014

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