Second Hard Drive for UNIX-pc; HwNote10

[Gil Kloepfer Jr.]

In article <7348 at chinet.chi.il.us> ignatz at chinet.chi.il.us (Dave Ihnat) writes:
[With regard to agreement that the HD upgrade and P5.1 upgrade be done in the
 same upgrade]
>This only if they're two separate upgrades, and CAN be acquired simultaneously,
>but need not be.  There are myriad reasons for keeping the upgrades separate,
>and virtually no good ones for doing them both on the same card/upgrade:

There are **VERY** good ones for keeping them on the same upgrade.  I will
start with those, since they answer many of your reasons:

	1.  The *only* real thing that the P5.1 upgrade performs
	    is the creation of the MCR2 register (physical I/O port)
	    which is used for a 4th head select bit, the second hard
	    drive select bit, and the signal to the OS that P5.1 is
	    installed.

	2.  Because of (1), having two separate upgrades means extra
	    circuitry to decode the address bits which ultimately
	    define the same port.  Highly unnecessary.

	3.  The simplicity of the second hard disk upgrade, minus
	    a lifted pin (can be left out if you never use the
	    second hard disk upgrade) is just as easy as the P5.1
	    upgrade.

To bring up an even more elementary point, the only reason why you'd want to
upgrade to P5.1 in the first place is for a 4th head select (for a big
disk drive) and/or the 2nd drive select.  You might as well kill two birds
with one stone than to keep opening the machine and fiddling with upgrades.

>1) Existing upgraded units.
>   A combined upgrade either would be unavailable to these units

This doesn't make too much sense in my book.  Are you talking about these
public-domain (sort of ;-) upgrades or commercial ones?  If the latter,
don't worry about it...there won't be.

>2) Complexity.
>   Keeping it a separate upgrade keeps the complexity down

Although not entirely wrong, the addition of the circuitry for the second
disk select isn't complex enough to make it separate from the P5.1/4th-head.

Furthermore, it means that you have to consider which upgrade will provide
the bit for P5.1.  Then you will end up having to install both if you
want to use the upgrade that doesn't do P5.1.  I believe you need P5.1
in order to use the second hard disk select bit.

>3) Cost
>   The cost of the combined upgrade will be greater than a single upgrade

Minimally.  The extra chips/sockets necessary for the second hard disk
will cost another $5-$8, depending on source.

As it currently sits, the P5.1 upgrade with 4th head select requires a PAL
which is not necessarily available to most folks (I had someone do it for
me as a favor).  The combined upgrade I proposed (and mentioned the $50
price) was that at absolute maximum to save my own neck should it be more
for someone else.  I *know* it can be done for a lot less than $50, and
even less for someone who has a good stock of chips available (I do a lot
of tinkering as a hobby, and keep a lot of chips around).

>4) Aesthetics.
>   Simply put, the two upgrades accomplish different purposes.  Keep them
>   discrete.

I already mentioned how it works at the beginning.  The aesthetic problem is
only in getting the wires from the cabinet.  If you don't use the 2nd drive
select, then you never plug the cable into the jack on the perfboard, it's
as simple as that.  In effect, you are getting more for your money, and
doing less to your motherboard over a period of time.

Inasfar as motherboard work goes -- I would feel a lot more comfortable
messing with my motherboard ONCE and leaving it alone, rather than connecting
wires to it TWICE and increasing the chance for costly damage.

I appreciate your opinion, but I don't feel that it is as substantial as
it appears on the outside.  Again, the two upgrades are closely related,
and they're almost meant to be done together.  I already have someone
who wants to work on a PAL design which will likely eliminate 4 TTL ICs and
could fit right on the motherboard like the current P5.1/4th-head
upgrade (although I would prefer to see the PAL on a separate piece of
perfboard with the appropriate disk driver chips, like John probably
does).  BTW: The combined upgrade now is a total of 6 chips, and about
thirty inter-chip connections [Vcc/GND included].  There are 14 connections
to the motherboard, which includes the power supply for the chips.  Seven
of the motherboard connections are required for the P5.1/4th-head upgrade.

Hope this helps to clarify things a little.

-----
Gil Kloepfer, Jr.          U-Net: {decuac,boulder,talcott,sbcs}!icus!limbic!gil
ICUS Software Systems      Voice: (516) 968-6860 [H]   (516) 746-2350 x219 [W]
P.O. Box 1                 Internet:  gil at icus.islp.ny.us
Islip Terrace, NY  11752   "Life's a ...  well, you know..."