View Full Version : Northstar MDS-AD Not Working

June 19th, 2015, 10:06 AM
Hello all,

I've recently been posting my successes here:

and here:

I have now acquired a Northstar double density floppy disk controller, MDS-AD:

I have been concerned, because, in my opinion, if you come across a single density Northstar floppy disk controller, such as my MDC-A4, you will probably acquire a good working board, because they were most likely working when they were removed from a system long ago in favor of a soft sector floppy disk controller, or a higher density Northstar floppy disk controller. Again, in my opinion, if you come across a double density Northstar floppy disk controller, it is more likely to be broken, because otherwise, why would someone have removed it from their system?

Yes, my fear has become true. When I received my board, I rushed to plug it in, in place of my single density controller. I powered up my system, filled memory with 00 (force of habit) and took a look at E800 to E83F, and I saw the code that looks like the Northstar boot code. YAY!

When I issued the command GE800 to initiate the boot process, nothing happened. Nothing. No motor turn on, no head load click, nothing. BAH!

I powered down, took out the double density controller, and put my original single density controller back in, and I re-verified that everything is working just fine.

I have downloaded the documentation that I can find out on the Internet that has the troubleshooting information in it for after one has built up the kit. The bad news for me is that all the ICs are soldered in except the 3 PROMs. This promises to be a difficult trouble shoot, if only because of that.

Does anyone here have any experience with troubleshooting a bad Northstar MDS-AD floppy disk controller? Any hints on what might be the most likely places to look first? Any hints on what broke the most on these boards?

Any thoughts or hints would be greatly appreciated.


June 19th, 2015, 01:12 PM
What did the drives do?

As I said in my original post, the drives did absolutely nothing at all.

What test equipment do you have?

I have a multi-meter, an oscilloscope, and a Saleae logic analyzer, if necessary.

Do you see any activity (scope) on the cable signals?

I have not yet done any probing, I was hoping for a few specific hints about where might be a good place to start.

Check the switch settings and tell us.

There are no switches to set on this interface board.

Are the DC voltages normal?

Since I can read the PROMs with my system monitor, I have been assuming that much of the board is actually working. I also am assuming that the DC voltages are correct, but I will check them when I get going.

Read Deramp's recent message about the slight register change between the SD and DD boards to see if it applies.

I thought that the register changes Deramp mentioned had to do with CP/M 2.2 vs CP/M 1.4 that I am running now. Otherwise than that, I am aware that the single density board has boot code at E800H and repeated again at E900H, while the double density board seems to have the boot code at only E800H.

Thanks very much for your thoughts. I will be posting back about what I find when I give it a go this weekend.


June 20th, 2015, 12:30 PM
Hello again...

JDallas, thank you very much for your attention and effort so far.

I currently have a pair of Tandon TM65-2L floppy disk drives attached to my Northstar MDC-A4 single density floppy disk controller by the 34 pin cable. These disk drives operate fine with the Northstar MDC-A4 in my IMSAI system. I recently acquired a Northstar MSD-AD double density floppy disk controller. When I put the MDS-AD double density board into my system in place of the original single density board, that is when things do not operate as expected.

So, for troubleshooting purposes, I do not suspect the floppy disk drives at all.

As it turns out, I mis-spoke when I said that nothing happens with the floppy disk drives when I execute the boot command. Actually, when I enter GE800 in my system monitor, there is a short (~1 second) hesitation, and then the drive access light comes on, on drive 1, as it is supposed to. After a few seconds (~5-7 seconds) the drive access light goes out, I suppose after the code times out from not doing a proper boot.

So what is not happening for me is that the drive motor is not being turned on. I guess that's a place to start looking.

BTW, I have verified the contents of the boot PROM from E800H to E8FFH by a disassemble listing that I picked up from Howard Harte's web site (I think that's where I got it, anyway).

As JDallas said, I believe that much of the board is operating properly. I know that my floppy disk drives are perfectly fine. Now to pull up the schematics and take a look at the drive motor command circuit...


June 20th, 2015, 02:19 PM
I spent much of the afternoon probing the MDS-AD board and scratching my head about why it was not working. I could see the "motor on" command go out on the proper pin but the motor did not turn on. I could see the "drive select 1" command go out on the proper pin, but again, no go. I spent a bunch of time swapping back and forth between my known working MDC-A4 single density controller and the MDS-AD double density controller and everything seemed to be the same between them, except the drive motor did not turn on when I have the MDS-AD board installed.

Then, all of a sudden, I saw that the drive motor WAS turning on with the MDS-AD board installed. Yes, all of a sudden, it is now working. I can't believe this. Could it be that there was oxide on the connector pins that I was plugging the 34 pin ribbon cable onto, and I've finally worn it away with all of my plugging and unplugging? That is my best guess at this point after seeing all the signals properly working on the board all afternoon and tearing my hair out wondering why the drives were not responding.

Thanks for listening. It appears that I may have a good, working Northstar double density floppy disk controller after all.