Well, this thread has been quiet for a while....well, wakey, wakey
When I first read this thread, I was intrigued (and somewhat blown-away) by MartinB’s excellent research and explanations of getting a 3.5” drive to work properly with HD disks (and I’m still in awe at how clever this mod actually is!).
Given that HD disks are more readily available than DD disks, I was determined to give this mod a go and hoped to give my beeb HD-disk compatibility! So, ordered a few 151’s and set the stage. Little did I realise the journey ahead of me at the time (not due to the mod itself, but due to the floppy drive I was using! More on that later).
(WARNING: This is a bit wordy, but I’m including details on the issues I encountered in the off-chance they might help anyone attempting this mod. This is all old news to MartinB, who’s way too familiar with the issues I had!
My 3.5” is an Alps DF-354 ripped from an old PC (a HP Vectra VL420). Also in my configuration is a 5.25” drive operating as Drive 0. The 3.5” connector had pins 10 & 12 flipped so figured I was good to go.
Incidentally, during various email exchanges with MartinB, his mod became unofficially christened as MBM (Martin B’s Mod, or Martin’s Brilliant Mod – take your pick. Regardless, we subsequentially referred to it as MBM. It needed an acronym and MBM sounds better than “the mod”
So I built the MBM (!) and fitted it, with one small change from Martin’s published schematic – as I wasn’t fitting a switch to my 5.25” I tied IC1 Pin 10 to 0v, via a 10K resistor. I had also made the change to the 3.5” drive to signal “HD mode” on pin 4 of its connector (0v with a DD disk inserted, otherwise +5v. In my case the DD disk was a HD disk with tape over its density-select hole).
The other slight variation in my setup is that my 1770 is Mark’s (RetroClinic) interface! Thanks to Mark for providing his schematic and MartinB’s input on figuring out where to make the clock track cut.
Thinking I had everything as it should be, I started to play with the MBM. It wasn’t working, so time to start probing. (My new scope had arrived a few days earlier – perfect timing, so the ideal opportunity presented itself to plug it in an give it it’s first mini-workout!).
I could see that IC2 Pin 5 was sending the 16Mhz signal to IC1 on Load Head, otherwise sending 8Mhz. All fine on IC2. However, IC1 wasn’t switching 16Mhz out to Pin 5 (1770 clock output). Therefore, time to check the input controls lines on IC1 (Pins 11, 10 & 9).
S10 East was correctly showing +5v with a HD disk inserted, thus picking up the “HD mode” signal from the drive and 0v on DD mode. I had already verified this before connecting the MBM. However, now with the MBM connected it was being pulled low to 0v, therefore the conditions for switching to 16Mhz didn’t exist. Disconnected MBM and the 5v was back. Hmmm…
Taking the 10K resistor out of the equation solved this. So, was there something in the 3.5” density-select switch unable to tolerate any additional load, thus pulling it low? I still don’t fully understand why this was happening but at least now I had a steady 5v in HD mode (& 0v on DD mode) with the MBM connected. Happy days!
Still didn’t have a working MBM! Onto the next input control line (Pin 10). As this was tied to 0v, this wasn’t a problem (remember, I wasn’t using a switch on my 5.25” so having this continuously in DD mode was fine).
The non-auto-switching issue had to be related to the last input line (Pin 9, Drive Select 0). This should have been showing +5v on drive 0 being accessed and 0v on drive 1 being accessed. Not so! It wasn’t showing any voltage on the meter and the scope showed frequencies hopping “all over the place”. Double-checked this was indeed connected to IC79 Pin 6 (and it was).
At this point it started to emerge that I had a drive identity crisis! Drive 0 was fine (*.0) but drive 1 only illuminated with *.3. Not having used the 3.5” previously on the beeb (no DD disks and no HD capability!) I never verified my 3.5” was working correctly as drive 1 before starting this (doh!). Shame on me!
With the 3.5” taken apart again, I could see it had some pads labelled DS1 & DS0. The DS1 pads were linked to Pin 12 on the drives connector and the DS0 pads (not linked in the pic below) were connected to Pin 10. My expectation here was that only Pin 12 would be used in the drives ID selection. In other words, with the DS1 pads linked (as in the pic) I expected the drive to operate as drive 1 and with the DS0 pads linked it would operate as drive 0.
Why would a drive use Pin 10 to signal its ID? Afterall, Pin 10 was “Motor Enable” in the beeb and I verified this by making the DS0 pad link and hooking it up to the beeb. Sure enough, the drives motor remained illuminated!
As I pondered over the Pin 10 & Pin 12 thing, it suddenly reminded me of my ribbon cable and the fact it had Pins 10 & 12 flipped! The penny dropped!
What if I used a cable with straight-through connections (no twist)? This would reverse the symptoms I was seeing above, namely, with the DS0 pads linked (i.e. Drive 0) I should be getting Drive Select 0 output on Pin 12, which is where I needed it.
So, I engineered (!!) a straight-through connection for the 3.5”, removed the DS1 link and connected the DS0 link. I also changed the jumper on the 5.25” to change its ID to drive 1. (If I was going to have a choice in drive ID’s, then I wanted my 3.5” to be my drive 0 and my 5.25” to be drive 1).
When I hooked it up, both drives were now operating just as I wanted them: 3.5” was drive 0 and the 5.25” was drive 1. Happy days (again!).
Now, back to MBM. With the scope on the 1770 clock speed input, I wasn’t seeing 16Mhz with the 3.5” when access a HD disk, but I was seeing the 16Mhz when accessing the 5.25”. What now?!
But as I had switched my 3.5” drive to be drive 0, I needed to also switch the connections on IC1 Pins 11 & 10, such that my DD/HD signal was now on Pin 10.
With the switch made, I could now see IC1 auto-switching from 8Mhz to 16Mhz when accessing drive 0 (3.5”) with a HD disk present and in 8Mhz mode with a DD (HD disk with the density hole taped over) disk present. It was also correctly operating in 8Mhz mode on drive 1 (5.25).
With the MBM finally working as expected, it was time to try to format a HD disk. It got as far as “…track 00” and hung. This should be working! Auto-switching betweeb 8 & 16Mhz was happening!
I eventually discovered that I needed to issue an *OPT80,1 command to force 80-track mode. I was using Watfords DDFS 1.54T. Why is this needed on the 3.5” when it automatically assumes 80-track mode on the 5.25” ? While it’s no big deal to issue this command before formatting a HD disk, I’d like to understand why it’s required! If anyone can explain this, I'd love hear your thoughts...
Anyways, a long story to get here, but the end result is a good one. I now have HD-capability on my 3.5”.
Mega thanks must go to MartinB who’s knowledge and expertise was fundamental to getting this solution working!
The number of emails back and forth between us could probably fill a few HD disks