Mountain Computer Expansion Chassis:

I was able to find this very rare piece of hardware for the Apple II recently. I was told it did work but i had to take a gamble on this as you never know if they do when they arrive. I know it’s for Apple II/II+ only so it won’t get much usage as //e is my main rig, but.. it is super rare collectable. You really don’t see these floating around anywhere, not even eBay.

I was suprised of it’s size, from the pictures i had seen i’ve always thought it was bigger. The design of the case clearly reasembles of Apple ][ with the wents in the corners.

Backside. The later revision had the backside panel not painted but baremetal.

There are few revisions known to exist. This one i got is first and early Revision A (14-000174-00) (C) 1980.

Serial 00306 that is quite low. The serial badge at back however looks different than the ones i’ve seen. Is it due it’s pretty eary one, or just exported model? Who knows.

I’m not sure how many of these were made, but there’s discussion for example at applefritter and estimates are somewhere in up to 1000+ pcs. I’ve seen later revision with serial 1025 (model : 01-00166-02, 220v 60HZ, 1,0A). Difference seems to be also with the enclousure, the back of the unit in later models seems not painted. The board inside is quite different as well.

Later revision product codes (from Mountain Computer Catalog 1981):
01-00166-01 (120V)
01-00166-02 (220V)

The powersupply interestingly looks like it’s US-model but the plug itself is clearly for the European markets as it’s two prong but rounded.

What there were?

-Expansion Chassis enclousure 
-Powersupply (clearly marked as 120VAC, 60Hz, 1.25A, 24VAC, but the wallplug is German, so i assume it’s 220V model.. but i’m not sure if i dare to test that..)
-Interface card (+50-pin connector cable, 16-pin header cable)
-The user’s manual was missing.

There are two different manuals for the product released (scanned pdf’s are available at

Mountain Computer Expansion Chassis (1981) : first revision

Mountain Computer Expansion Chassis (1982) : new revision 

What it is used for?

The Expansion Chassis is ment to be used with Apple II and II+ as external enclousure to house 8 more cards you wanted to use, not in the same time of course. It was even possible to have even up to four Expansion Chassises installed. You can switch either by hardware (switches/jumpers) the setup or by sofware. More likely this kind of setup would be used by developers or industrial needs and not in general household.

The setup:

There is few led’s in front of the chassis indicating what mode the Chassis is at:
GREEN = power on/off
RED = selected on/off

Jumpers & Switches:

Jumpers on interface card:

Settings of multiple expansion chassises (1 to 4)

Settings how the Slot 0 is been used.

The interface card:

The interface card is inserted to any of the Apple II/II+ slots, also in slot 0 if you’re not using the 16k language card. The 50-pin cable needs to be installed between the chassis and the interface card and also the 16-pin cable is attached to location “J2” and the chip removed first.

Installing cards:

There is lots of potential usages for the Expansion Chassis and the cards inserted to either the Chassis or in Apple II. Scenarios with DOS, CP/M, Pascal etc. Possiblities are almost unlimited.


The expansion chassis, even power chord attached, will not receive power unless the Apple II is powered up. There is select mode and deselect mode that can be altered with either the button on the chassis or by software at any time:

-Integer basic : POKE -16352,254 (select), POKE 16352,255 (deselect)
-Applesoft basic : POKE 49184,254 (select), POKE 49184,255 (deselect)
-Monitor (CALL -151) : C020:FE (select), C020:FF (deselect)

It’s not suggested to use the casette port as the switching uses the same memory space locations. The selection can be done from Pascal language as well.

More detailed usage scenarios are explained in the user’s manual.


If only similar product would have been made (would that be possible?) that would work with Apple //e as well. I don’t see myself using this so much as i don’t keep II/II+ in my desk as daily reqular setup but it’s historical as well and it’s collector’s value is essential.

There’s project to reverse engineer it at applefritter and at:



FinApple 2018 (c)
Apple II Forever.

p.s. I might trade this to nice Bell & Howell Apple II+ with backpack and black floppy drive. Contact me.