CoCoSDC for TRS-80 Color Computer part 1

See also: part 1part 2part 3part 4, part 5 and part 6.

2014/12/30 Update: Tim Lindner corrected me. CoCo drives started out as double density, not single density. Corrections have been made. That means the 80 track 5 1/4″ drives I had were quad density. Thanks, Tim!

After a series of issues with the U.S. Postal Service with multiple shipments to multiple addresses, I finally have received my CoCoSDC and enclosure kit. As previously mentioned, CoCoSDC is a floppy drive controller for the Radio Shack TRS-80 Color Computer that emulates the original disk controller but instead of accessing physical floppy drives, it accesses data on an SD memory card.

First, let’s discuss a bit of CoCo disk drive history…

The Radio Shack TRS-80 Color Computer (later nicknamed “CoCo”) was introduced in July 1980 as a 4K computer that hooked to a TV set. The computer had a TV output (like an Atari 2600), and ports for a cassette cable (for hooking to a cassette recorder to load or save programs), serial port (for hooking up to a printer or modem), and two joystick ports. A cartridge port was provided for running software from “Program Paks” or future hardware expansion.

One such future hardware expansion was the Disk Drive controller (catalog number 26-3022) released in 1981. Although there were already some third party disk drive products, once Radio Shack released an official implementation, it would set the standard and other third parties would release their own compatible clones in coming years (J&M, Hard Drive Specialists, etc.).

The disk drive interface plugged in to the CoCo’s cartridge port and had a 34-pin edge connector coming off of it. A ribbon cable would then connect to standard 5 1/4″ floppy drives. The original Radio Shack unit came with a full height, single sided, 35-track, belt-driven 5 1/4″ floppy drive which provided 156K of storage on a single double density diskette. Up to four single-sided disk drives were supported by the interface and the new Disk Extended Color BASIC which was contained on a ROM chip inside the cartridge. (Or, three double sided drives, but no one had those yet.)

Electornically, the interface could support double sided drives with up to 80 tracks (720K). This would later allow the use of 720K double density 5 1/4″ floppy drives, as well as 3 1/2″ drives (introduced in 1983, and made popular by the 1984 release of the Apple Macintosh). Thank goodness for compatible hardware standards (back then).

I found a good rundown of the Radio Shack floppy drives at Techno’s CoCo Floppy Page. Here is a brief summary:

Over the years, it appears Radio Shack released five versions of the disk drive interface. The original required 12V to operate (which the first Color Computer model provided, but later CoCo 2 and 3 models did not provide). Using it on a CoCo 2 or 3 required using a Multi-Pak expansion device, which plugged in to the cartridge port and provided four selectable cartridge slots (as well 12V power for older devices). After that, there was the FD-500 (26-3129) which was a 5V version of the original, followed by the FD-501 (26-3131, released in 1986 with a shorter cartridge) and the FD-502 (1986, 26-3133).

Besides circuit board changes, each release of the disk interface came with different floppy drives. The first two came with vertical full-height single sided drives. The FD-501 and FD-502 came with half height drives in a horizontal case with a slot to add a second drive later. The final FD-502 release was special, as it included a double sided drive, even though (for compatibility reasons), the Disk Extended Color BASIC still treated it like a single-sided, 35 track device. Patches were created to make Disk BASIC use more of the disk space, but disks written in that format could only be readable by other users running the same patches. Alternative operating systems like Flex and Microware’s OS-9 also could make full use of the storage.

On my personal CoCo setup, I eventually had a standard 40 track double sided single double-density (DSSD DSDD) for my primary drive, and two other 80 track double-sided double quad-density (DSDD DSQD) 5 1/4″ floppies for storage. This gave my OS-9 BBS a massive 1.8 megabytes of storage! (I could have had up to 2.1mb if I had an 80 track drive for my boot disk, but I wanted one drive that was 100% compatible with stock formats). Later, I replaced my 5 1/4″ double density drives with 3 1/2″ drives (though I always had to keep a 5 1/4″ around, since the standard format software was sold on was still that).

Over the years, other store options became available with the introduction of hard drive interface cartridges from Radio Shack and third parties. I used several in my days, starting with a Burke & Burke interface that would use RLL or MFM hard drives, then moving to a KenTon SCSI interface and RGB-DOS (patches to Disk Extended Color BASIC which let you access the hard drive as if it were up to 256 floppy disks).

The introduction of the Iomega Zip drive (with swappable diskettes that stored 100mb each – wow!) was a problem, since the KenTon interface did not generate hardware parity which the Zip drive required. Because of this, I ended up using the competing SyQuest EZ-135 drives (which were faster, and stored more, but ultimately lost the format war to Iomega).

My final hard drive interface was the SuperIDE from Cloud-9. I never actually had an IDE hard drive hooked to my computer, but I did make use of the CompactFlash slot on the interface and used a CF card for my hard drive.

That’s quite the evolution from the original 156K floppy drive of 1981!

But I digress. This article is about the CoCoSDC. As you can see, over the years, floppies became less important as hard drive solutions became available. Many advanced CoCo users might only have two floppy drives (one for compatibility and booting, and possibly a second to make floppy backups easier). The need for 80 track floppies went away once low-cost hard drive options were available.

So why am I so excited about the CoCoSDC? It finally replaces the need for a physical floppy drive for anything but reading old media. Darren Atkinson designed the CoCoSDC to emulate the original CoCo floppy controller, so when it is plugged in to a CoCo, any software designed to access the floppy hardware will still work. This may seem like an obvious statement, but over the years, attempts to bring hard drive support to old systems have always had compatibility limitations. For instance, while Disk BASIC could be patched to allow the user to access virtual floppies on a hard drive, any software that was written to access the floppy controller interface directly would still try to do that, and would never run through the hard drive interface. Only software that used the native Disk BASIC floppy access routines had any hope of running from a virtual floppy drive, and even then, while RGB-DOS patched basic to let you have 256 virtual floppies, even compatible software might only let you type in “0-3” for the drive to use so it might never let you use those extra drives.

CoCoSDC solves this by pretending to be the original Western Digital 1774 drive controller chipset to the CoCo. This makes it effectively 100% compatible with any software that ever ran from a Radio Shack floppy drive. However, there is still one limitation I can see. CoCoSDC can only map two floppies at a time, so if you actually had software hard-coded to require three or four floppy drives to run, you would be out of luck. (I believe this limitation is caused by the available RAM in the processor on the CoCoSDC.)

In part 2, I will discuss why all of this is incedibly important.

9 thoughts on “CoCoSDC for TRS-80 Color Computer part 1

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