Premium/Premium+ floppy drive cable

[IanMcLean]

I have actually asked Ewan this by private message on Facebook already so apologies firstly to Ewan, I was having troubles logging in and seeing the New Thread button on the forum.
Now that I seem to have that sorted I will put the question here as well, as it may be a bit easier in a forum post to sort this out then via messenger.

Whilst my Premium+ has SD card emulation I would like to add some real floppy drives to it. I have an enclosure very similar to the original Microbee 56k dual floppy enclosure that I would like to use.
I have a working Copal F-5002 5.25" 360k DSDD floppy drive on the way and I would like to try this out. I am thinking of making the second drive a 3.5" 800k DSDD once I can track a working one down.

However, I am not sure how the 34-way floppy drive cable should be configured. I have noticed from a post some time ago on the MSPP that the 256TC had pins 10-12 swapped on the floppy drive cable for drive A: and drive A: went on the end of the cable, so for drive B: these pins were not swapped. A PC has pins 10-16 swapped over.

So what is the correct configuration for the cable on the Premium/Premium+ for 1 and 2 floppy drives? I would assume just pins 10-12 swapped on the end of the cable for drive A: as for the 256TC. I'm guessing this based on my assumption that the drive pinout is the older variant before HD drives came about which use pins 10-16 for drive select. Thought I better check with those more knowledgeable though before I risk damaging anything.

[ChickenMan]

Well on my PP+ I have used a straight through cable. I've had a Gotek as B: and booted a 5.25" as A: once to copy some files across and worked well, with the 2 SD card drives as C: & D:. I was not aware of any twisted cables in a 256TC, I must check mine.

On my normal Prem 128k its always been a straight through Shugart cable.

[IanMcLean]

Yes, thanks ChickenMan for the quick reply and for confirming this. This was what I was told this morning too by Ewan (he was very quick in getting back to my private message). I had forgotten that the older double density drives have a drive select jumper and that the cable is indeed a straight through. The twist on the 256TC drive cable is something I saw on an old MSPP post - someone was using dual 3.5" floppies and had pins 10-12 twisted on the end of the cable for drive A: Maybe this was something non-standard.

[Ernest]

I can confirm that the 256 TC can’t work with a crossover cable, it uses the Shugart interface which is a 1-1 standard 34 pin cable.
The signal pin usage is different for the IBM interface that uses a crossover cable to save on manufacturing configuration work.

Ernest

[IanMcLean]

I can confirm that the 256 TC can’t work with a crossover cable, it uses the Shugart interface which is a  1-1 standard 34 pin cable.
The signal pin usage is different for the IBM interface that uses a crossover cable to save on manufacturing configuration work.

Ernest

Thanks Ernest and ChickenMan for the quick replies! I'm very pleased I checked on the forum with this and have received the same information from both of you and Ewan confirming the right way to do this. I am glad I did not rely on what I saw on the MSPP forum for a 256TC restoration, or it wouldn't have worked.

So that just leaves one question, I wonder what this is all about?

https://www.microbee-mspp.org.au/gallery...play_media

[ChickenMan]

Looking at those pics, those 2 floppy drives are not the same thats in my unit and I cant see any D0, D1, etc drive selector on them. They also have Beige front faces and not the original Black that I've seen in all the 256TC's I've ever seen. So I assume they are both B: drives with the drive select twisted to allow the drive on the right side to appear as A: and somehow the bees able to sort that out. The 256TC boots to a menu if there is no floppy in A: drive. You then have the option to press SHIFT F1 and it can boot from B: drive. I've put a Gotek in as B: in my 256TC which allows booting from a real floppy in A: or the Gotek in B:.

[Ernest]

Looking at those pics, those 2 floppy drives are not the same thats in my unit and I cant see any D0, D1, etc drive selector on them. They also have Beige front faces and not the original Black that I've seen in all the 256TC's I've ever seen.  So I assume they are both B: drives with the drive select twisted to allow the drive on the right side to appear as A: and somehow the bees able to sort that out.  The 256TC boots to a menu if there is no floppy in A: drive.  You then have the option to press SHIFT F1 and it can boot from B: drive.  I've put a Gotek in as B: in my 256TC which allows booting from a real floppy in A: or the Gotek in B:.

Hi Ian,
ChickenMan is 100% correct, this was very much a none standard way of getting around the issue with latter model 3.5" dives which were hard wired as IBM B-drive or if you prefer Shugart drive 1.
Without the ability to change the drive selection by jumper on these newer drives then drive selection must be changed externally, in this case by swapping DS0 and DS1 by applying the unconventional twist. The use of the standard IBM twist cable just would not have worked. 
If you have no option but to use the method above that RedskullDC used, then do not simply dissassemble the connector and twist the cable over and reconnect. I would strongly recomend cutting the cable just short of the old connector create your twist and perform a fresh assembly of the connector to the cable. Once a displacement type connecting to the cable is disturbed it can easily end up with a break to the conductor giving all sorts of intermittant issues.

Caution must be taken though when using these modern derives for the Microbee, they are fine for reading the old 80 track DSDD disks but a very different story when it comes to writing. If you only use the written disks on the same system they were written with you should be fine, however compatibility with the old drives can be a problem with narrower the head width, so the disk may not work on other drives (very hit and miss).

Ernest

[someone]

The IBM PC used drives with only 2 drive select lines and 2 motor enable lines.
This fact causes some confusion on how to connect up drives to a microbee.

The microbee disk controller was designed to be used with drives with 4 drive select lines DS0-DS3 with DS0 being used for A: drive and DS1 for B:
DS2 and DS3 are actually were used by special in house microbee systems to handle multiple drives formats  e.g. 400K 3.5, 800K 3.5, 5.25 400K and 5.25 800K.
(At microbee we even had the 3.0" floppy disk drives like the Amstrad CPC and Sinclair ZX Spectrum working but chose to use the 3.5" drive as used by the Apple Mac.)

The 34 pin Floppy disk drive cable is an IBM PC quirk that was done to expedite PC manufacturing.
It simplified their PC manufacturing process by allowing the Drive select jumpers on the floppy disk drives all be set to Drive #A and have the drive address determined by the cable assembly.

The IBM PC floppy disk adapter supported signals two drive select signals DSA(pin14) and DSB (pin 12)  with pins 10 and 16 assigned as corresponding Motor Enable signals.
IBM also shoved a peg into the connector socket pin 5 and removed the corresponding connector pin as a key to prevent insertion of the cable incorrectly.

Watch out for the variations with the microbee Floppy disk controller schematics.  The DRAM coreboard uses the correct even numbered pin annotations for the disk drive control signals and odd pins as GND return pins.  Some early schematics have the connections flipped with the disk drive control signals on the odd numbered pins and GND returns on the even pins.
The net result is just the position of the connector key being either on the top or bottom side.  It was not uncommon to file off the connector key to make things fit.