papa@pollux.usc.edu (Marco Papa) (12/24/87)
It is Christmas, so I decided to make somebody a favor. This is my
little present. I swear I do not use the word "multitasking" in the
rest of the message :-)
HOW TO CONNECT DRIVES TO THE A2090
A TUTORIAL
By Marco Papa
Felsina Software
1. Introduction.
I bought an A2090 Hard Disk controller from Commodore and one of the
"supported" drives for it, a Seagate ST251, 40Meg MFM ST506 drive from a
Texas mail order outlet, and I spent 1 night trying to figure out
how to connect them. This tutorial will hopefully save some time
to other Amiga users.
2. Cables.
Neither the A2090 nor the Seagate drive came with cables. It turns out
that the reason is that ANY IBM XT/AT cables will do. One can buy the
cables from a lot of mail order outlets. One, which I found was very
good, is listed at the end of this memo.
There are basically two types of connectors on the controller and the drive:
a 20-pin connector (J2) ans a 34-pin connector (J1).
The controller has 3 of them, arranged as follows:
top of the controller board ---------------------------------------
------ ------ ++++++++++
20-pin 20-pin 34-pin
Drive 2 Drive 1
The 20-pin connectors go directly from the controller to the individual
drive. On the other hand, there is ONLY 1 34-pin connector that is "shared"
by two drives, when you have both of them attached to the controller. As
all IBM controllers, the A2090 supports up to 2 drives.
The 34 pin connector can be of two types:
Type 1 is a straight 34-pin with only ONE connector at the drive end. It turns
out that it is VERY difficult to get this cable.
Type 2 is a cable with 2 connectors at the drive end, separated by about 6
inches, and with a few signals "twisted" after the first connector. It looks
like this:
|--------------------------|---------\/------------|
|--------------------------|---------/\------------|
^ ^ ^ ^
34-pin 34-pin "twist" 34-pin
To Controller Drive 2 Drive 1
3. The Drive options
Any ST506 drive comes with a few option jumpers. One is called "drive select"
and has usually 4 positions. For a controller like the A2090, that supports
(like the IBMs) 2 drives, only Select Drive 1 and Select Drive 2 ara valid.
Moreover a "terminating" resistor must be present ONLY on the "last drive" in
the chain. If you are installing 2 drives, remov it from the first one in the
chain.
4. Using 1 NON-twisted cable
If you are installing only one drive and are lucky to have found a cable of
type 1 (only one connector end and non-twisted), you should set the jumper
to DRIVE SELECT 1 and connect the 20-pin cable to "20-pin Drive1" as from the
above figure. Most drives that you buy mail order will come pre-set with
DRIVE SELECT 1 (mine did).
5. IBM setups.
The complications comes up when you get, as most everybody will, a cable
of type 2.
Drives that come "pre-setup" for an IBM (XT/AT) come with DRIVE SELECT 2 on
BOTH drives. The reason for this is simple:
* If you are installing one drive, you'll connect it to the END of the
34-pin connector, that is the "twisted one". The twist will change the
Drive SELECT 2 to a DRIVE SELECT 1. In IBM-lingo this is "drive C".
* If you are installing a second drive, you'll attach it to the internal
34-pin connector, the "non-twisted" one, and this will really be used
as DRIVE SELECT 2. In IBM-lingo this is "drive D".
If you try to connect a SINGLE drive to the internal connector, and set it
as Drive SELECT 1 (as if you were using a cable of type 1), it will not
work because the unconnected cable end acts like an antenna and will pick
up noise. As a result the drive will not be properly recognized.
So when you are installing a single drive with cable type 2, you MUST:
1. Change the jumper on the drive to DRIVE SELECT 2
2. Attach the drive to the END (twisted) connector
When you are adding a second drive, you MUST:
1. Change the jumper on the drive to DRIVE SELECT 2
2. Attach to drive to the internal (non-twised) connector.
There are other few things that could go wrong.
The connectors at the controller end must have pin 1 at the "upper-left".
This is usually visible as either a red stripe on one of the two sides of the
cable, or a little arrow (almost invisible) on the connector plastic.
Nothing can go wrong at the drive end since there is only ONE way to stick
it in: you'll see what I mean when you take a look at it.
6. WHERE DO I BUY?
One place that I found offers a good cable at a good price is:
Walker Electronics
3521 Hacienda, Dallas TX 75233
(214)339-4916
ask for:
XT Hard disk cable, 34 pin Daisy for 2 HDD $ 6.95 (+$2.00 packing).
Do not ask for an AT Hard cable set. This is the same as the XT cable
PLUS 2 extra PC floppy cables (which you do not need on the Amiga).
I hope this will be useful to somebody. If I had know about this beforehand
it would have saved me time and hassle. The A2090 manual is definitely
lacking in this respect. I am VERY satisfied with the performance I get
from the A2090+Seagate ST251.
Happy Holyday.
-- Marco Papa
Felsina Software
P.S.:
I do multitask a LOT on the Amiga. So, I lied :-)