Leaving out a lot of stuff over the last year that probably no-one
wants to read about, I finally built some 16MB SIMMs for the Mac
IIfx. Suffice it to say that I took a circuitous route to getting
my boards made and there were many layouts along the way.
The IIfx uses an oddball 64 pin SIMM which is nearly identical to 30
pin SIMMs, *except* the data-out and data-in busses are separate.
This allows the memory controller to latch the data for writes,
signal the CPU that the transaction is complete in two cycles, and
then proceed with the actual write to RAM. Since the write data is
sitting in a little buffer-to-SIMM cul'de'sac it does not interfere
with other use of the data bus.
Memory chips which are X 1 memory chips (e.g. 16M X 1) have separate
data in and data out pins, though in most SIMMs, they're simply tied
together and run to the SIMM data bus. I had a small supply of 16M
X 1 chips on hand, so first I built some 16MB SIMMs with eight 16M X
1 chips each. That was very straight-forward, no surprises and
worked fine. That is not the interesting part.
Next I built some SIMMs out of 16M X 4 memory chips, of which I have
a much larger supply. The problem with 16M X 4 chips for this
application is that they do not have separate data-in and data-out
pins. They have combined D/Q pins. This presents a problem as
simply tieing them together would mean that the buffered writes would
interfere with other activity on the data bus.
Most of the discussion a year ago was how to get around that problem.
I finally tested it. This is the (I hope) interesting part.
My first idea was to use a pair of octal buffers per SIMM, such as
the TI SN74ABT241. Four of the eight buffers are controlled by an
active_low OE and four of them are controlled by an active_high OE.
This makes it convenient to use one buffer per 16M X 4 chip. I
planned to use WE_ to control the buffers. The active_low buffers
would route data from the D/Q chip pins to the SIMM data_in pins.
The active_high buffers would route data from the D/Q chip pins to
the SIMM data_out pins.
Before trying it I realized that the flaw in this plan was that when
the output of the memory chips is high-Z (high impedance/no signal)
the buffer would still be trying to drive some deterministic output
onto the data bus as long as WE was high.
My next thought was to invert CAS and logically AND it with WE to
control the data_out buffers. The data_in buffers would still be
controlled by WE alone. That way the data_out pins of the SIMM would
only be driven when there was actually a Read taking place.
However, the timing on this looks wonky. Data out from a read is
usually held a while after CAS goes high again, but this scheme would
cut off the data_out signal (switch back to high-Z) very shortly
after CAS goes high.
My third idea was to ask this list if there was some component that
would just pass the signal along, including high-Z signals.
Someone pointed me at a family of components which led me to FET Bus
Switches. So an octal bus switch such as the SN74CBT3244 looked
like a good option. The only problem with the SN74CBT3244 is that
both OEs are active low (each OE controls four of the eight
switches). I wish there was a SN74CBT3241 with one active low and
one active high OE. So, I simply controlled the OE for the data_in
pins with the WE signal and controlled the OE for the data_out pins
with the inverted WE signal. SC-70 packaged inverters are *tiny*.
Ultimately, the SIMMS based on 16M X 4 chips with two SN74CBT3244
octal switches controlled by WE and inverted WE switching between
data_in and data_out worked.
The other two configurations using the SN74ABT241 octal buffers did
not work. I tried both control configurations. I tried two
different assemblies. And when the second one did not work, I
converted it to the SN74CBT3244 configuration just to test whether
the memory chips were good. After the conversion to the workable
control circuitry the SIMM worked. So, unless I received a bad
batch of SN74ABT241 chips, I'd say that the Bus Switching scheme
works and the Buffer scheme does not work.
When I tested my first SN74CBT3244 controlled SIMM I placed it in the
D24 - D31 SIMM socket, while the other three SIMMs were the
conventional ones built from 16M X 1 chips. It almost booted, but
not quite. Moving the SN74CBT3244 controlled SIMM to any of the
other three sockets resulted in it working, but it would not work in
that one socket.
Hoping that the problem had something to do with combining SIMMs of
such different components, I built a second SN74CBT3244 controlled
SIMM. With that one installed in the D16 - D23 socket, the original
in the D24 - D31 socket and the other two sockets with 16M X 1 based
SIMMs the machine booted and passed memory tests.
Similarly the machine works fine with three and four of the
SN74CBT3244 controlled SIMMs installed.
Any idea why it would not work when just one SIMM was different and
installed in that one socket? Much of the IIfx's peripherals seem
to use only 8 bits and the 8 bits they use appear to be D24 - D31, so
I would guess it has something to do with that.
Also, I've always heard/read that when a machine needs four SIMMs at
a time, one should install SIMMs of the same manufacture and
composition. But why is that true, electrically and logically?
If the SIMMs meet timing specification, shouldn't they work together
even if they're built of wildly different components? What
difference does it make if some SIMMs are 60 ns and others are
effectively 70ns if the machine requires 80ns and so they're all
meeting spec? Or why should it matter if some SIMMs have a bit more
drive current or sink a bit more current on writes than others, as
long as they are all within specfications? Yet it seems that it
matters. Why?
I could understand it if some memory chips were 13 X 11 addressed and
others were 12 X 12, but these are *all* 12 X 12, so that is not the
issue here. So why should combining SIMMs built from 16X1 chips
with SIMMs built from 16X4 chips not work in some configurations?
Yet when placed in like groups they work fine?
Jeff Walther
What do you do when you buy a small lot of items, a piece that you wanted is not included (shown in the picture and stated in the inventory), and the seller does not respond to your questions (but you paid very little for the lot and there was a couple extra things included)? What would you people do?
> From: "Teo Zenios" <teoz at neo.rr.com>
> What do you do when you buy a small lot of items, a piece that you wanted is not included (shown in the picture and
> stated in the inventory), and the seller does not respond to your questions (but you paid very little for the lot and
> there was a couple extra things included)? What would you people do?
For some reason, a number of people feel that spam filters are okay on their
ebay email address. Others seem to feel that it is okay to check their email
every week or two. You haven't said how long you've been waiting, but I would
suggest at least a week. Personally, unless it was something important, I'd just
forget it, and leave neutral feedback saying the piece you wanted wasn't
included, but there were extra things.
On 14 Jun, 2006, at 05:17, cctech-request at classiccmp.org wrote:
>
> tom:
>
> Can you provide a source where I can purchase 11/16 inch wide
> spools of
> paper (prefer yellow) for the reader mentioned above
>
> Thanks
>
> David C. Masterson
Not yellow, but as you are in the states, try here:
http://www.wncsupply.com/paptape.html
I have a source in the UK for yellow ONE inch tape, and they
can make 11/16 to order, but minimum batch is hundreds if not
thousands of rolls.
Roger Holmes
> Have y'all had much luck acquiring vintage computer books/info via
> library book sales?
Absolutely. Silicon Valley is one of the best places to be for this.
This is where "vswmoretp" on eBay get some of their stuff.
There is a whole used book buying/dealing subculture around the book
sales out here.
Get to know the people at your local library that handle the book sales.
If you are after mags like Datamation, you may need to talk to them
about saving them for you, since they probably would be recycled
otherwise and not even put up for sale, since periodicals don't sell well.
--- Teo Zenios <teoz at neo.rr.com> wrote:
> What do you do when you buy a small lot of items,
a
> piece that you wanted is not included (shown in th
e
> picture and stated in the inventory), and the sell
er
> does not respond to your questions (but you paid
> very little for the lot and there was a couple ext
ra
> things included)? What would you people do?
>
A similar thing happened to me a while ago.
However, my situation was slightly different
in that the seller apologised for losing one
of the items and asked me if I was happy with
the extra items he sent instead.
In your case, if the seller doesn't respond I'd
open up a dispute. If the seller still doesn't
repond then post negative feedback.
Regards,
Andrew B
aliensrcooluk at yahoo.co.uk
As a several boxes of floppy disks slipped and fell, the question occurred ...
if you have a disk with only a vague idea of what computer it might go to, is
there an easy way to find out what computer made it without trying it in a
number of systems? Is Teledisk or Dave's program likely to be useful? And what
about Apple disks?
And to make it "easy", assume the disks all look like soft-sector disks.
Lyle,
It has been a while since you updated the Classiccmp list with the status of
your project to make TSX+ available to hobbyists. Can you tell us where the
project is at the present time?
Regards,
Stuart Johnson
the only time a totem-pole output would have an issue if it had to fight another output - one low and one high. As long as you are driving using a single output that has to deal with the resistor network, you should be just fine. The interface you are describing is basically the same as on a 8" disk drive.
best regards, Steve Thatcher
-----Original Message-----
>From: Chuck Guzis <cclist at sydex.com>
>I was thinking of driving the lines with an LS540. which has an I(OL)
>of 24 ma per output. What I was curious about was if the low-level
>sink current is sufficient, will I run into problems with the totem-
>pole pullup? In other words, do I really need OC drivers?
>
>Thanks,
>Chuck
>
