• Please review our updated Terms and Rules here

5.25 and 3.5 on the same cable

tcoulon

New Member
Joined
Nov 14, 2015
Messages
3
Location
Switzerland
Hello all,

I'm new on this forum. I did a search on my subject but got no match.

Here is my problem: I am in the process of restoring an Amstrad PC 1512 (XT class, 8086 at 10 MHz). I've managed to install an ST-225 20MB drive salvaged from a no more working IBM, re-low-level formated it (it was a long time I hadn't used Debug...) and found images of the original install disks. I've managed to copy these images to the hard disk (by shutting off the IDE controler of an old Pentium PC and stuck the MFM controler in the ISA bus - to my amazement it did work!

Now, to ease data transfer, I'd like to convert the pentium (it's a Packard Bell) to a machine with hard disc, DVD ROM, 3.5 and 5.25 floppy. And there is the problem: I have a cable that has three connectors: from the motherboard connector you get: a 3.5" connector, a 5.25 connector, then the twisted cable and another 3.5" connector.

I don't manager to get both drives working: I set the 3.5 as A: (1.44MB) and the 5.25 as B: in the BIOS. That way, I can access the 5.25, format floppies and read/copy files. The 3.5 however does not work (tells me drive not ready). If I disconnect the 5.25 the 3.5 works (note that I have to physically disconnect it, just disabling in the BIOS does not work.

So my machine more or less does the job, but I have to alternatively connect or disconnect the 5.25. Am I missing sometĥing ? I've spotted a cable on ebay with the IDC connector _after_ the twist, could this solve my problem? The 3.5 drive has no jumper visible, and the 5.25 has only one that seems to define if first or second drive (I can only access the floppies if I set it to second, at least on my cable.

Any advice is welcome,

Thierry
 
1. Both drives need to be set to the second drive select (e.g. DS1, not DS0--but be careful--some older drives have DS1 as the first)
2. Cabling Motherboard->Drive B->twist->Drive A
 
Welcome to these forums.

Drive termination is also important. Terminate per the diagram at [here].

THank you for this information - I think that may be the clue (I remebered temination for SCSI but not for floppies). However, as close as I inspected the drive I can't find anything like a terminator (at least a removable one)- There are four positions I can put a jumper on (labeled DS0 to DS4). DS1 clearly makes it drive A, DS1 works as drive B (but with A unreadable). DS2 and DS3 did not seem to work differently from DS1.

Looking for information about teh drive (QUMETRAK 142) I was pointed to this information:
"The IBM PC floppy disk drive bus includes the facility for up to four drives to be daisy-chained, with device addressing being controller by pins 6, 10, 12 and 14.
Initially, jumpers were used to manually select the device ID (0 through 3) with a flat ribbon cable.
Another option is a 'twisted' ribbon cable, where the cables serving pins 6 to 14 are reverse between the first and sector drive connector, thereby providing 'cable select' addressing for two devices (only). Using this method, both drives should be jumpered to drive 1 (DS1), with the drive connected to the first (untwisted) connector being drive 1, and the drive connected to the second (twisted) connector being drive 0 (DS0)."

So, _Another_ seems to mean that you can't have both. I understand this as meaning I should _not_ use the twisted part. I do have an untwisted cable, but it requires a "flat" connector on the floppy controler. So I guess I should connect the 3.5 on the second connector, before the 5.25, set the 360K as A and the 3.5 as B, but none of my 3.5 floppies seems to accept to work that way...

I guess I have to search for an untwisted cable (if it exists) or live with my plug/unplug system.

thanks anyway,

Thierry
 
It's possible that your cable is damaged. I had a similar issue and it turned out to be a bad cable.

Twisted vs untwisted cable is not going to make a difference. The twist just changes DS1 to DS0 on the last drive. They were introduced for ease of assembly, so all floppies could be set to DS1 and the position on the cable would set the drive letter.

Normally you would set both drives to DS1, put the 5.25 on the middle connector and the 3.5 on the end connector. In the BIOS set A to 3.5 and B to 5.25. I think this was your original setup.

If this is not working, possibly there is a termination problem. Put the 3.5 in the middle and the 5.25 on the end. In the BIOS set A to 5.25 and B to 3.5.

If that doesn't work either, try another cable.
 
A service manual for the QumeTrak 142 is available from here. To quote from the service manual:

TERMINATOR

The input line terminator is a 150 Ohm DIP resistor module which is at the
factory plugged into the DIP socket, location RNl, on the P,CB. This
terminator must be present in a single drive, or in the last drive of a daisy
chain.
 
The input line terminator is a 150 Ohm DIP resistor module which is at the
factory plugged into the DIP socket, location RNl, on the P,CB

Thanks, with your information I found it (I had not expected a chip that big). however removing it does not change anything, so I'll try another cable. I'll tell the result when it arrives.

Regards
 
Well, I did find a PC1512 reference online and discovered something interesting. I was mislead by your characterization of "XT compatible". The Amstrad is very close, but not totally compatible. One key issue is that if motherboard link LK8 is installed, pin 34 READY is used. This is not what the IBM 5160/5150 do. So if LK8 is installed, remove it. Otherwise, you'll get a bunch of "Drive not ready" errors on 1.2 and 1.44 drives, which use this line as a "disk changed" signal.

Most 3.5" drives have integral "weak" terminators (usually about 1-2K), which should be sufficient for your application. It's not important that the 5.25" drive have them.
 
Looking for information about teh drive (QUMETRAK 142) I was pointed to this information:
"The IBM PC floppy disk drive bus includes the facility for up to four drives to be daisy-chained, with device addressing being controller by pins 6, 10, 12 and 14.
Initially, jumpers were used to manually select the device ID (0 through 3) with a flat ribbon cable.
Another option is a 'twisted' ribbon cable, where the cables serving pins 6 to 14 are reverse between the first and sector drive connector, thereby providing 'cable select' addressing for two devices (only). Using this method, both drives should be jumpered to drive 1 (DS1), with the drive connected to the first (untwisted) connector being drive 1, and the drive connected to the second (twisted) connector being drive 0 (DS0)."
The floppy cable situation with the IBM PC is shown at [here]. The twist also affects the 'motor on' signals. The drives are configured to only turn their spindle motors based on the state of the 'motor on' line.

So, that is the situation with the IBM PC, however, whilst a lot of clones use the same mechanism, some do not. Some use a flat cable, with one drive set to DS0 and the other to DS1, and each drive is configured to turn its spindle motor when the drive is selected.

Member vwestlife discovered that his PIII Compaq DeskPro used a similar mechanism to the IBM PC, except that the 'twist' had already been done on the motherboard. So a flat cable is used for a single drive. And a twisted cable used for two drives, however, because two twists are in place, the situation is like the bottom cable in the diagram at [here]. That results in B: being at the end of the cable, and A: in the middle,

Ideally, to save time, a member of these forums with a PC1512 will categorically state that the two-drive situation in the PC1512 is xxxxxxxxxxxxxxxx.

The earlier thread at [here] indicates that the PC1512 has a flat cable that is soldered to the motherboard.
 
I finally located the PC512 service manual. Modem7 is connect--the floppy hookup is not the same as the PC XT--it's the "flat" cable (when the motor line is activated, both motors go on). Beats the devil out of me why the port mapping calls for separate motor enables--probably has to do with real XT compatibility.

I'm also correct in that you want link LK8 removed. So, a flat cable--you can use the current one with a 5.25 to 3.5 floppy adapter (used to be part of the mounting kit for mounting 3.5" drives in 5.25" bays):

adapt3.jpg


Or crimp a 34-conductor IDC connector onto your present cable (take care to identify where pin 1 lies). No desoldering necessary.

Set the "A" drive to the first drive select (usually DS0) and the "B" drive to the second drive select (usually DS1).

You should be fine--just take care to remove LK8.
 
I may be misreading his original post, but I think he wants to put two floppy drives in his Packard Bell (Pentium), not his Amstrad PC 1512.
 
I may be misreading his original post, but I think he wants to put two floppy drives in his Packard Bell (Pentium), not his Amstrad PC 1512.
You're right.

This is what happens when you get old; easily confused. And forgetful too. I have just remembered that there is a post over on the Vintage Computer Forums that I need to reply to.
 
I missed that too--and I'm no spring chicken either. Topic drift. In my defense, the OP did start out by talking about his PC512.

The PB thing should be straightforward--if the PB is capable of supporting two drives. Some cost-reduced machines were equipped with support for only one drive.

If that's the case, you'll have to resort to something such as a Microsolutions Backpack floppy drive--or find an ISA controller card that has 2-drive support.
 
There is one more thing to consider here. Since you are using a QumeTrak 142, you will have an issue with pin 34 on the cable. The 3.5 inch drive uses pin 34 for DISK CHANGED. The QumeTrak 142 use pin 34 for READY. My solution was to modify the cable so that pin 34 is not connected at the QumeTrak but is connected on the other connectors. The following is a reply from Modem7 in another thread that might clear things up.

"In the early days of the 5.25" floppy interface standard, pin 34 was optionally used for a READY signal (floppy drive is ready). Some 360K drives generate it and some do not, and some (like the JU-455-7) have a jumper to select the desired behaviour.
The IBM 5150 and 5155 and 5160, as supplied by IBM, do not care what is on pin 34. If you look at the IBM floppy controller supplied in the 5150/5155/5160, pin 34 is not connected. Accordingly, the floppy portion of the motherboard BIOS doesn't care.

With the introduction of the IBM 5170, pin 34 is used by the supplied floppy controller, and 1.2M drives are required to supply a DISK CHANGED signal on pin 34.
The rules for the IBM 5170 are:
* 1.2M drive: Must generate a DISK CHANGED signal on pin 34. The controller and motherboard BIOS expects that signal from 1.2M drives.
* 360K drive: Generate no signal on pin 34, not even a READY signal. Disconnect pin 34.

Like the 5170's floppy controller, the XT-FDC controller has pin 34 connected. The 5170 rules probably apply. I'm guessing that your Qume 360K drives generate a READY signal on pin 34, and that was confusing the XT-FDC, until you disconnected pin 34."
 
Last edited:
Funny, since the 142 uses an edge connector, I just put a bit of Scotch tape over the finger.

In reality, however, any BIOS that allows for configuration of a 360K drive will know to ignore the READY pin. It has ever been thus.
 
In reality, however, any BIOS that allows for configuration of a 360K drive will know to ignore the READY pin. It has ever been thus.
My experience with IBM 5170s is that if a 360K drive is added that generates READY on pin 34 (and the BIOS is set for 360K), the drive will not work. If I then disconnect pin 34, the drive works.

Scott Mueller, in his Upgrading & Repairing PCs book indicates:

f4574yes634y.png


Is he suggesting a hardware-level issue, i.e. the floppy controller chip is designed (or configured) for Disk Change only.
 
Your source is mistaken--on the 5170 and above, port 03f7h was added to the hardware. Bit 7 of this address provides the current status of pin 34 of the floppy interface. The 765 controller (or simulation if the 765 core is part of a larger LSI device) is not aware of this bit, period. Note that the PC 5150 and 5160 do not implement this port in the standard hardware. Yes--on some 720K 3.5 drives and on many 360K 5.25" drives, this is the "/READY" status. Well and good--the adaptation by the 5150 team to adapt the 765 (which was really intended for 8" drives) to their purpose was practical, I suppose.

In fact, the 765 does have a "drive ready" input--and it's actually used on the Amstrad PC512 noted at the top of this article. If you don't want to use it, you have jumper LK8 to disable it. The 5150 people chose to simply tie it high for some inexplicable reason and provide their own drive-select mechanism with external logic.

If you consult the Shugart SA-400 OEM manual, you'll see that pin 34 (and pin 32) is NC. Even on the double-sided SA-450 OEM manual pin 34 is designated as "spare signal line". This, I suspect, is what caused the 5150 team to ignore the "/READY" signal.

So, where does pin 34 "Disk Changed" enter the picture?

Well, you can find a bunch of Microsoft and IBM trouble reports that say, "I changed floppies and the system didn't see it". In fact, this was a problem going back to the 8" days--which is why CP/M implemented the "CSV" scheme in its disk interface--a simple checksum on an implementor-selected number of directory entries in an attempt to detect a disk change. Some 8" drives can, in fact, be jumpered to provide a "disk changed" signal--and many came with an optional door lock solenoid to prevent the user from opening the drive at the wrong time. So hardly new or original on 5.25" drives.

The problem is really a software one--if you have a system that has mixed 360/720 "ready" type drives and 1.2/1.44 "disk changed" drives, you have to condition your reporting of the BIOS Int 13H error 6 by the type of drive--on 360/720 drives, you ignore the flag; on 1.2/1.44 drives, you report the status. (FWIW, the "disk changed" flag is reset by seeking on the drive--and it's possible to time things on some drives such that a disk is changed in middle of a long seek and that status is missed completely.) But the bottom line is that pin 34 is a status-only pin and can be used or ignored without affecting the operation of the FDC proper--it's strictly a software matter.

Smart software (drivers and/or BIOS routines) know the difference. As a matter of fact, in the original 5170, location 40:8F includes a flag that indicates if the disk change line is supported by the current drive.

Stupid software (drivers and or BIOS routines) doesn't know the difference and gets bogged down.

As an aside, last night, I was refurbing a 3.5" drive used on a Mitsubishi PLC that runs MSDOS. Pin 34 is "/READY" and pin 2 is "/DISK CHANGED". It seems that you can have your cake and eat it too.

Don't trust everything you read. I did some technical editing on contract for the Osborne-McGraw Hill series back in the 90s. The people who write them often know very little detail and rely on their technical editors for accurate information. Get an editor or author that slips up and you've got trouble.
 
Back
Top