emmo@moncam.co.uk (Dave Emmerson) (08/10/89)
First, an apology. Due to a misunderstanding between Harry & I, Harry
recently posted that we might run a second batch of the 2/4 Mbyte
expansion board. Regrettably, that's a no-go. The batch we had made were
cheap as a returned favour from the PCB makers. Also, we do not wish to
damage those companies who make their living from selling extras for the
home computer market. Their presence is vital. As an aside, I DO think
they should offer THEIR bare boards for the DIY people, who will otherwise
resort to 'piggybacks' or prototyping boards. If anyone else can get a
cheap batch run, and will undertake to sell at around cost price, the
photography is still available. Remember, this board is for ZIGZAG drams.
<change hats..>
I've been following the multitasking debate with some interest. I disagree
generally about the MMU being ESSENTIAL, Sinclair's QL (forget the grotty
keyboard & microdrives) made an excellent job of it using nothing more
than a 50Hz interrupt & a good OS. Sure, a rogue task can scribble all
over everything, but even a MMU wont stop bad code from causing total
system failures, only one particular type, and only for a limited number
of tasks.
Incidentally, somebody (not me, I'm off for a week [sighs of relief] )
should post a short article explaining to the less hardware-oriented
types, what a memory management unit does, so's they can all join in
(or at least understand what's being said).
<change hats>
The participants in the decoupling capacitor debate have both omitted to
mention that the most critical parameter when dealing with fast changing
signals is the INDUCTANCE of the connections between the IC's power pins
and the decoupling device. I read the results of tests made with an American
product (Q-CAPs ??) which were large thin capacitors designed to fit under
the chips, connecting directly to the pins with NO inductive path between
the capacitor and the pin. As a result, you could reduce the capacitor by a
factor of 10 and still see an improvement. I forget who actually makes them,
but RS sell them in the UK, one is their ref 126-269. The point is that
they can also be retro-fitted ON TOP of the chip, if you have problems.
I was taught selection of the value of a damping resistor is a function of
the impedance of the track and the rise time of the signal. John M Logajan
is quite correct, DON'T change it unless you alter the track. Hitachi
(as I recall) suggest that PCB track width and thickness be selected so that
this will normally be within the range of 25 to 50 ohms, but I doubt if Atari
chose 68R randomly. Piggybacking chips will not impact greatly on the
track impedances, adding flying leads will.
Continuing the discussion prompted by jbww@ukc.ac.uk (J.B.W.Webber)
>(ever done a recursive directory listing on a network of Suns?), but,
Yes, and symbolic links make it even messier but I wouldn't want to do away
with them...
Seems like what you need is a nonvolatile ramdisk - How about bubble
memories, I haven't heard much about them for a while.
Looks like you may have to be patient. Still, it didn't take long to
get from a 1K ROM, 1K RAM system to where we are today...
No more sounding off for a week, I promise!
Dave E.JOHNBARNES@ENH.NIST.GOV (11/15/89)
REPLIES TO VARIOUS SUBJECTS:
1). From "Bob Marley" (address given as BAILEYS%.... in digest no
625- RE: Tech Specialties Catalog.
I had a bad experience with Tech Specialties on a disk drive kit.
Colleagues of mine have had bad experiences with deleiveries of
"Tower cases". I would not buy anything from them. Some maga-
zines no longer accept their advertising.
They failed to deliver their products in a timely manner and then
failed to support them once delivered.
2). To "Marty" with the boot problem - The symptoms you describe almost
always go away when you replace a defective .ACC file with a good
copy. You may also be experiencing marginal reading ability from
your disc drive. Strip away all .ACC's and start over. Autoboots
can also cause this problem. Strip them out and reload them
on at a time until you find the one that causes the problem. Also
make sure that your machine has been turned off long enough to clear
RAM.
3) Request for Uniterm on Disc - Get hold of a copy of Current Notes
Magazine from your local store. Uniterm is in their PD library.
If it is not listed in the latest issue just check with the
publisher at the address given in the magazine. I think the disk
number is around 302.
4). To Mustafa Thamer: There are a couple of decent command shells.
The one I use most often is DO_IT! from QMI. In the PD world
there is GULAM (look for it in the NET World) for those who
insist on unix-like obscurity. There are a couple of simpler
ones like PCOMMAND available from Current Notes.