The hostadaptor in the MegaSTE and STACY is only able to use one harddisc with unit number SCSI 0 without parity. With this little modification the hostadaptor (fully Atari-compatible) is able to adress two harddiscs (compatible to the Megafile 44 hostadaptor, which means that you can use the regular harddiscdriver!).
All you have to do is to add two wires and a 1 KOhm resistor! The one wire goes from chip U4 pin 12 to chip U8 pin 1, the other from chip U8 pin 2 to pin 44 of the seconds (the new one!) harddisc SCSI cable.
Both harddiscs are connected to the same SCSI cable (therefore you have to squeeze a second 50 pin liner onto it) and the second harddisc has to be jumpered onto SCSI ID 0 (yes, you are right: both harddiscs have ID 0!). Cut the SCSI-cable between the first and the second harddisc, only and exclusive at pin 44 (directly behind the 50 pin wide socket of the first harddisc is recommended). Now the second wire from chip U8 pin 2 has to be soldered at this vein (to the vein which goes to harddisc no. 2)! After you have done this, solder the 1 kiloohm resistor from pin 1 of the resistorarray RP3 (pin 1 is the one which points to the description REV. 2 or REV. 3) to pin 2 of chip U8.
Now you need a Y-cable for the harddiscs voltage and to put the computer into operation with the two harddiscs. If necessary you must reconfigure your harddiscdriver so it can find the new harddisc on system-bootup (Please refer to the drivers' documentation).
Beware: The hostadaptor has only been built for short SCSI cables! The cable should never (!!!) be any longer than 20 centimetres/7.8 Inch. For larger cablelengths there is another termination used than the hostadaptor has (impossible to change!).
On the other hand, you only need short cablelengths in the MegaSTE and the STACY (particularly in the STACY there is only room for one harddisc when you also want to use the discdrive) and even when you use a caddy 20 centimetres/7.8 Inch are more than enough! Furtheron NONE of the harddiscs is allowed to be terminated, so FUNDAMENTALLY remove the ending terminators otherwise the drivers on the hostadaptor were overloaded and may disappear in a cloud of smoke (no joke!).
In the end: The device adresses:
dip 1 dip 2 dip 3 --------- dip-switches on the hostadaptor on on on only ACSI-unit 0 (Atari default) off on on ACSI-unit 0 for the old harddisc ACSI-unit 1 for the new harddisc off off on ACSI-unit 1 for the old harddisc ACSI-unit 2 for the new harddisc off on off ACSI-unit 2 for the old harddisc ACSI-unit 3 for the new harddisc off off off ACSI-unit 3 for the old harddisc ACSI-unit 4 for the new harddisc
These instructions were produced and tested with the highest care.
I cannot estimate how this list, instructions or modifying of your computer corresponds to your abilities. The person who carries out the above-mentioned modifications always bears responsibility. In case of doubt let a friend or even your computer dealer do the modifications. I cannot take responsibility for irregular done works (e.g. computer is out of order) because I cannot verify if your abilities corresponds to the demands to do the modifications in a correct way.
This is especially when you work with voltage or currents. Electrotechnical laymen have to STAY AWAY from voltages higher than direct voltage 120 volts or alternating voltage 50 volts (in Germany!). In other County's/Country's take a look at the law of order for electrical units and regulations.
Electrotechnical laymen are all persons, who did not get equivalent instructions or who do not have an electrotechnical education.
hint: these instructions can be found seperately under `MEGAE_HA.TOS` on bulletin board systems (german version only).