Re: Oric Drives

On 2005-07-23, Twilighte <twilighte@xxxxxxxxxxxxxx> wrote:

> That cable looks perfect Andre! And the price is not too bad, infact
> i am willing to spend a little to get a decent Oric setup.
>
> Cool idea about the soft reset, since i don't have a spare button on
> the front for it.
>
> About connector, i would hope to put a socket on the front of the pc
> for the keyboard. Something like a 25 way D.

I used something like RS 468-866 for the case (male) and RS
446-355 for the cable (female). Cheaper versions are available.

> I found some nice angle brackets to mount the 3" drive in a 5.25" bay,
> although do you know of any easier way to have a nice escution other
> than cutting out one from a blank plastic plate?

There are commercially available cradles (plate + brackets) for
3.5" but I've never seen any for 3".

> The other problem i am a little concerned about is the ATX power
> supply.
> 1) Is their a good link to what all the connectors are?

I think I posted a link to the ATX spec here a month or two ago.

> 2) How do i know what sort of PSU it is?

There are only three sorts on the market right now : AT, 20-pin
ATX and 24-pin ATX. AT PSUs must be hard to find now.

> 3) Are their any rules when taking power off them, such as are ATX
> psu's switch-mode ones or balanced loading?

As Jani said a few weeks ago, switching PSUs need a minimum load
to regulate reliably. It seems that the higher the power rating
of the PSU is, the higher the minimum current is so prefer a
"small" PSU (200 W or less if you can find one).

ARM users have reported problems with their ARM systems, which
draw about 5 W. The Oric mainboard itself draws about 3 W. So,
assuming the PSU needs 10 W, you'd have to sink another 7 W. You
can do that with a resistor of R = U/I = U(P/U) = U²/P = 5²/7 =
3.57 ohms.

For the 12 V line, assuming 10 W again, you'd need to sink 10 W.
The resistor value would be R = U²/P = 12²/10 = 14.4 ohms.

There are several choices of resistors. I think that the easiest
would be to buy these two :

RS 160-916 3.3R 50W £1.78
RS 160-938 12R 50W £1.78

Note that these are designed to be heat-sink mounted so screw
them to the case of the PC. Position them on the outside and
vertically so as not to hamper convection. Stagger them so that
the top resistor is not in the hot air flow from the bottom
resistor. Use thermal grease.

Heat sinking is important. The data *** says that the
resistors should be able to sink 20W on their own but they'll
get dangerously hot. Even with the case acting as a (mediocre)
heat sink, expect them to get as hot as 70°C. If that's too hot,
I have enough aluminium scraps to make you a pair of heat sinks.

Those resistors are big (30x73 mm). If space is a problem, the
25W version is shorter (RS 160-708 3.3R 25W £1.45) but I don't
recommend it.

As an alternative to resistors, Jani has suggested light bulbs.

> 4) would the main oric board supply bypass the 9v curcuitry to
> directly power the +5v?

That's what I would do. I would remove the 7905 and connect the
PSU there (0V to pin 1 of 7905, +5V to pin 3, pin 2
unconnected). But please double check this with more competent
people.

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
(Counterfeit: wid@xxxxxxxxxxxxxxx rihohyc@xxxxxxxxx)
"J'baiserai la France jusqu'à ce qu'elle m'aime." -- Un rappeur
.