Radio Shack TRS-80 Model II 5.25" Dual Floppy Modification |
| Date Added: July 06, 2009 06:37:08 AM |
| Author: Link Directory .TV |
| Category: Computers & Internet: Vintage Computers |
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 A few years ago I found a TRS-80 Model II and 6000HD stuffed in the back of warehouse. Well, actually, at the time I found them they were mostly parts in boxes and covered with a thick layer of dust. Not having the time to fiddle with them when found I cleaned up what I could and stored them. I finally made some free time and was able to resurrect the Model II to the point where it was asking me to insert a disk. I found TRSDOS and CP/M here: Daves Old Computers - Disk/Software Image Archive Sadly I didn't have any 8" media. Finding 8" disks in good condition wasn't easy and when you did find some they were not cheap. The only 8" drive I had was inside the TRS-80 and I didn't know if it even worked. Doing some more investigating I discovered these sites: Utilizing 5.25" or 3.5" media on your Tandy/Radio Shack computer system. Using a 5.25" 1.2Mbyte floppy disk drive in place of an 8" drive. Having spare 5.25" 1.2MB floppy drives I went to work making a cable as described in the above pages.  With a newly created CP/M disk (It wasn't easy finding a PC that had a floppy controller that could write single-density but after some searching through my old PCs I found I had a Pentium Pro based IBM 365 PC that could). I disconnected the internal drive and attached my new cable and 5.25" drive to the external floppy bus. It didn't boot. After triple checking my cable I was convinced I had it correctly made. Examining the floppy controller I found that the DS0 (Drive Select 0) wasn't even connected to the external floppy bus (Grrr!). Once attached to the internal bus I was finally able to boot CP/M. Yeah! Some measuring made me realize two 5.25" drives would fit nicely into the 8" bay. This is where I ran into more trouble with the floppy controller. I added a second floppy port to my cable, attached a second drive and reconfigured CP/M. When I attempted to format a new disk the drives light would come on (controller was selecting the drive) then CP/M would error with: DRIVE NOT READY. Hmmm. More fiddling rendered no successful outcomes. After a discussion on The Vintage Computer Forums and constructing a second cable for the external floppy bus (with an edge card connector so I could use it internally) I finally had 2 drives working! Note: It's much easier to connect a 34 pin cable to a 50 pin connector than the other way around.  Another note: It turns out that I have an older revision floppy controller in my Model II (I was informed that dual internal 8" drive systems existed so I shouldn't be having a problem). The controller I have is a slightly older version than those found in the TRS-80 Models 16 & 16b and as such refuses to work with 2 drives on the internal floppy bus. When I find out what modifications need to be made to the older controller I will document them here. So now all I needed to do was mount the new drives. Removing the 8" drive from the Model II was no easy task as I had to remove the front of the case. Having a long flat head screw driver and a nut driver with extension bars was a requirement. With the drive removed I was left with a steel bracket that was about 1/4" lower than the edge of the opening.  I had some scrap plastic that was serving no useful purpose and was thick enough to allow the 5.25" drives to sit above the edge of the opening. Using that plastic I constructed a mounting plate for the new drives.  I drilled out all the holes (I still don't know why as all I needed was 4) you can see in the image at one point I had the drives upside down (the lever to close the drives would have been at the bottom - I wanted them at the top).  With the drives mounted I hit another snag. The 8" floppy drive power connector didn't supply the +12 volts that was required to power the 5.25" drives only +25, -12 and +5 volts. I didn't want to run extra wires from the internal power supply and possibly overload it. The 8" drive did have an AC line that powered the 120 volt motor used to spin the disk. I found a power supply from an old external hard drive case that fit nicely into the open space behind the drives and a matching power connector.   The final task was to fill the extra space above the 5.25" drives. Using more of the scrap plastic and an old XT/AT style floppy mounting bracket I made a filler panel that attaches to the top of the floppy drives. I placed the plastic over the hole, slid the bracket forward so it would firmly hold the plastic and applied some epoxy. Viola!    |
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