Re: Restoring a NorthStar Horizon, problems with SRAM board

On May 23, 6:59 am, Dave.Dunfi...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
(Dave Dunfield) wrote:
A couple of quick questions though. Dave, would you email me the
assembler source for that ROM only monitor you mentioned earlier?

Done (assuming the email address on your posting is valid - if not,
please send me the best one to use).

After reading the MDS-A floppy drive controller manual, I noticed a
short note. What are the symptoms if the SA-400 floppy disk head
carriage gets out of the spiral groove track? Does it make the floppy
drive behave in a halting or choppy manner? How do you know if you
have the problem and how is it fixed?

When the head positioner comes out of the spiral grove, the head
doesn't seek (at all) - it stays on whatever track it's in, or between
tracks if thats where it happens to be (it rarely "comes out of the
groove" on it's own - usually it gets knocked out). To the software,
the drive will either get errors (if between tracks), or load the wrong
track - Frequently, during a seek the drive will "fix itself" (see below),
but now it doesn't match the track number the system thinks the
head is on as some seeks have been lost. This is particularily bad
with the NorthStar system because it does not record the track number
in the sector (so it doesn't detect this) - fortunately, if your drive is in
good shape, the head should never come out during normal
operation.

The head positioner is simply a spiral groove on a disk which is
rotated by the stepper motor, and a ball on a spring that sits in the
groove - it's fairly easy to knock out, although I don't think I've
ever encountered one that did so without help.

To fix it, just slide the head carriage back and forth until it "clicks"
into the groove. you can also simply seek to track 0. the disk will
rotate until the groove comes under the ball, then it will move the
head. Couple of things to note:

1) If the head is already at track 0, and out of the groove (rare),
the drive won't step on a home (it's already at track 0). If
track-zero is indicated before you start, seek out 35 tracks,
then home the head to track 0.

2) You may need more than 35 steps to reach zero, as the first
few will be ineffective until the ball goes into the groove.

Dave

--
Dunfield Development Services http://www.dunfield.com
Low cost software development tools for embedded systems
Software/firmware development services Fax:613-256-5821

Dave,

Thanks, I got the monitor and am reviewing it. Today has been good
news. I have gotten the CPU and motherboard to respond to a simple
boot ROM program.

The boot ROM program is simple but represents a lot of advances. I
replaced the on-board 2708 EPROM with a DS1220 and shim socket
circuit. I had some problems with my EPROM programmer but the good
folks at www.willem.org helped out and got it working.

I made a new auto-jump DIP header for the CPU board to jump to the
boot ROM at $F800. I also replaced the first serial port
configuration DIP header for a more standard "modem" hardware
interface. Now it is 1200 bps, 8 bits, 1 stop bit, and no parity.

The boot ROM is as simple as can be. All it does is initialize the
motherboard IO and then starts writing "0" to the first serial port as
fast as it can. What is nice about this program is that I can verify
that the serial port is configured right and also trace the signal
back from the serial port all the way to the CPU board.

Next, I am going to try to get the boot ROM to repeat whatever input
it gets from the serial port. Once I can establish that the serial IO
works, then start working on the ROM monitor.

This week while I was gone, my oscilloscope arrived and it seems to
work fine. I can see signals like the 4 MHz clock on pin 6 of the
CPU. Also, I now have my logic probe so that will help some too.

I managed to get to the electronics surplus store yesterday and bought
a bunch more spare parts including some real 74LS241s. I replaced the
74F241s with them and nothing worked so I put the 74F241s back in. I
can now get very close to a floppy drive boot but it is still very
inconsistent. My best strategy now is to get the CPU board,
motherboard, and SRAM board debugged before starting in on the FDC
board.

So good news. Hope all is well for you and thanks for all your help!

Andrew Lynch

.

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