Bootable Model III Disks...

Tony Duell ard at
Fri Mar 5 15:25:15 CST 2010

> > > Nope, not gonna take it back.
> > > In my circle of users in Brooklyn, EVERYBODY with a Model I had a
> > > doubler if they had disk drives. EVERYBODY.
> On Thu, 4 Mar 2010, Tony Duell wrote:
> > It is extremely unscientific to stick to a statement when you have clear
> > evidence to the contrary.
> I haven't seen any clear evidence that he every hung out with anybody
> without a doubler, so his statement could be technically true.

Oh, I am quite happy to accept that all the people he assoiciated with 
back then who had M1s had doublers. But that was not the original statement.

> > > and had lots of trouble reading disks.
> Among the people that _I_ hung out with, everybody had already solved that
> long before the doubler became available.
> > This is soemthing I can't explain. I have a 'stock' M1 EI, using the
> > internal data separator of the 1771. I never had _any_ problems with it.
> > It read disks with no isseus at all. I had many more problems with the M3
> > (first of all that infernal tapewire cable between the CPU board and the
> > disk controller board, and in fact I had to replace the 1793 chip in the
> > M3 at least once). Oh well...
> I wonder if there were any differences in the Expansion Interface, other
> than the power supply?

AFAIK it's not the PSU that changed, not even between the European and US 
models. THey all rand off the same type of 'brick' as the M1 keyboard/CPU 
unit. You could fit the 2 bricks inside the EI to save space, the 
European ones were larger (and in metal cases) so you had to dismantle 
the case and remvoe a redundant plastic tray to do this. Who cares about 
the warranty anyway :-)

> Around here, there were lots of EI problems unrelated to the FDC.
> RS did a "free" "upgrade" of EIs to a "buffered cable" between the CPU and
> the EI, then did an "upgrade" to add an additional DIN cabel, then another
> "upgrade" which included switching back to an unbuffered cable.

This all relates to the DRAM timiog. The DRAM system needs 3 accurately 
timed signals, RAS/, cAS/ and Mux (this being the control input to the 
address multiplexer). They are all produced and used on the M1 processor 
board for the intenral DRAM. ANd they were brought out to the expansion 

The original intention was to feed those along a ribbon cable to the EI. 
This AFAIK never worked propperly. The next idea was to fit a buffer PCB 
in the calbe ot clean up those signals. Then to use a screned DIN cable 
to carry them. And findally to only use the RAS/ signal from the M1 CPU 
board and to geenrate the other 2 signals inside the EI using a delay line.

My EI is one of the last design. I have just dug out the scheamtics that 
came with it, and from waht I can see there were no changes to the disk 
cotnroller section. Thre is no external data separator, the XTDS/ pin of 
the 1771 is pulled high (this pin is low if you have an external data 
separator and high to use the one built into the 1771 I think).


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