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Reviving a Morrow Designs DJ/DMA fdc

T

Ttpilot

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Nov 8, 2020
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100
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South Dakota USA
Now that the North Star Horizon motherboard and power are sorted, along with the Compupro CPU-Z and RAM-17 boards, it's time to try to revive the DJ/DMA fdc. Actually, I have two of the boards, so I'm hoping I can get at least one working. I'm starting with the one that I bought off Ebay a year ago. The board works by taking over the bus on startup or reset, using it's onboard Z80 to load the first sector off a floppy and boot from that.

Well, it doesn't boot. The drive doesn't even come on, even though I know it's in working order. The two 7805 regulators are working okay, and the chips are getting power. I checked the Z80 on my breadboard test rig and it's working okay. I checked all the 74 series chips on my TL866 II programmer and they all passed. There are some chips I can't check, such as the 81LS95 and 81LS96. I have a couple of spares of those, so I swapped them out. There is a 2732 EPROM. The programmer can read that, plus the one on the other board, and the stored data matches between them. There are several other chips, including:
  • 82S105 (labeled DJDMA 2C) (at location 2C on the board)
  • 16R4 PAL (labeled DJDMA-2B) (at 2B) This can be read on the programmer as a 16G8 GAL; it looks like it consists of nothing but 1s. The programmer saved it as a .jed file. Tomorrow I'll compare it with the 16R4 on the other board
  • 6305 (labeled DJA-3D-2) (at 3D)
  • 9401PC (at 4C)
  • 6305 (labeled DMA-11C) (at 11C)
  • 6301 (labeled DJ-12B) (at 12B)
I'm at a loss as to what should be on these chips and if they're in working order. Does anyone have a way of reading these? Also, does anyone have the data from working boards I could compare with?
 
I found the PAL and PROM contents on archive.org. The 9401 turns out to be a CRC chip, so it's probably okay. The 6301 and 6305 are bipolar ROMS. The PAL16R4 I think I can replace with a GAL16V8 and can program with my TL866 II Plus. Now I'm on the hunt for someone who has some of those chips and can program them
 
I was able to read the bipolar ROMS on the DJDMA board (actually, both boards) following a method outlined by Damian MacMillan. When I compared them to the ROM dumps I found online I get the following results:

6301 ROM at 12B on the boards: The chips on the board seem to match the ROM dump
6305 ROM at 3D on the boards: The chips on the board seem to match the ROM dump
6305 ROM at 11C on the boards: The chips on the board seem to match the ROM dump, even though the contents look to be empty

I've been told that the 16R4 chip at 2B is a registered PAL device and can't be read. My set of ROM dumps includes a dump of that chip, but I have no way of verifying it. However, seeing as how the others seem to have survived, I'm guessing the 16R4 is okay too.

I'll have to keep hunting for the problem

djdma-6305-@11c.jpg

djdma-6305-@3d.jpg

djdma-6301-@12b.jpg

Edit: bad grammar
 
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I checked all the 74LS chips. They were okay, but I found a 74LS221 in a 74LS123 socket. I switched in a 123. I set all the cpu switches, set the ram board to 64k and risked it. The thing worked!

80D94561-97DC-4261-9124-791D1955F126.jpeg

I was able to boot, see disk contents, run some programs:

5C46A861-F0B0-4343-B8F0-45FCBDD26583.jpeg

Eventually, this happened:

17343387-4EB6-407F-8EED-34F6B5D0B9C3.png

It‘s like it forgot how to read the drive. A reboot fixes it for a while. I replaced the cable and switched some drives back and forth, but it’s still happening

I‘m open to suggestions…
 
The behavior might indicate a memory problem, bytes (or bits) fading over time. The CP/M CBIOS might be becoming corrupted, rebooting reloads the CBIOS.

I am considering the possibility of a similar configuration for a N* system I am working on. I have read the DJDMA manual that is on the web and I am a little unclear on the boot process. Where did you get your boot disk? Is it hard or soft sectored? How is your CPU card configured? What is it set to for power-on/reset jump? It is not clear to me whether it should be set to 0x0000 or 0x0038. The fact that it loads NOOPs starting at 0x0000 would seem to indicate that a few instructions may get through when the reset switch is released. Anyway, I assume that you have J2 jumpered B to C on the DJDMA? Your photo seems to indicate that your console is attached to the N* serial port. Is that correct? Do you have an alternative serial connection on J3 or have you shunted it? If you could make an image of this boot disk, it would be extremely helpful.
 
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The behavior might indicate a memory problem, bytes (or bits) fading over time. The CP/M CBIOS might be becoming corrupted, rebooting reloads the CBIOS.
Click to expand...
I put the North Star fdc back in and booted into NSDOS to use the monitor. I used that to run memory tests most of yesterday. No errors. I also loaded a huge chunk of RAM with 11s and let it sit overnight. I scanned through and no changes. I guess RAM's not an issue, at least not on the RAM17 board. The DJDMA does use a couple of small RAM chips. I'll keep working on it

My original boot disk is long gone, I think. Reaching back over 40 years in my inner file system backups, I seem to recall that I bought an 8" Shugart drive with the DJDMA, and my CP/M distribution disk was for that. I later made bootable 5.25" versions. I sold that drive a couple of years after I got it (Jeez those things were expensive!).

Anyway, I'll be glad to make an image as soon as I can. Right now I'm having trouble getting software transferred onto the system disk from my computer. You're correct that my system uses the North Star serial port. The CPU-Z card is configured to jump to 0000h on power up and reset. That board can only be configured to jump to 256 byte boundaries. J2 is jumpered 2-3 and the J3 port is jumpered on pins 2-3.
 
Excellent! You should be able to image them with Dave Dunfield's hard sector tools and I should be able to recreate here with the same. Thanks!
 
Excellent! You should be able to image them with Dave Dunfield's hard sector tools and I should be able to recreate here with the same. Thanks!
Click to expand...
I think that'll have to be put off a while. I also have a North Star MDS controller. When I put it in I experienced some of the same problems. I also noticed that my CPU card would no longer run a PROM mounted on it. I installed a couple of different monitor PROMs on it with no luck. I'll start hunting through that to see if I can get it functioning

It's so damn frustrating trying to troubleshoot these problems when you can't be sure any board in the system is running right. It's like having a boat with no anchor

EDIT: Thanks for the reference to Dave Dunfield. I've seen the web page but never noticed the North Star tools. I'll have to try those out!
 
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