Click here or click onto the photo for a full size version of this picture. The 70-R21 was IBM's first true 486 machine. It came out immediately after Intel released the first 486 CPUs in quantities. IBM took advantage of the fact that the up-to-now high-end Model 70 had its CPU on a daughtercard, so they only had to design a new CPU module, which was simplified by the higher integration of the i486 CPU compared to its predecessor. The 70-R21 therefore looks mostly like all other model 70s, you have to take a close look at the IBM label at the front to see the tiny '486'... IBM also offered the 486 module as an upgrade for people who already had a Model 70 with a 386 module and refunded the value of the old module - except for the optional i387 coprocessor... Click here or click onto the photo for a full size version of this picture. Click here or click onto the photo for a full size version of this picture. One has to remove the plastic frame holding drives and cards in their place to take a closer look at the CPU module. Compared to the old 386/25 module, it's rather empty - the cache, cache controller and coprocessor are already integrated. The heatsink is my own addition - the 486DX25 used in this machine is a very early chip release and becomes quite hot. It works fine without a heatsink when the cover is closed, but without the cover, the air doesn't flow along the CPU any more and you run into trouble quite quickly... Click here or click onto the photo for a full size version of this picture. A closer look onto the CPU module reveals that the 486 CPU module was a real quick-hack - note the amount of corrective wires ;-)

Processor: Intel i486DX @ 25MHz Coprocessor: integrated into CPU Cache: 8K internally in CPU Memory: 8 Mbytes (options range from 2M to 8M) Bus: 3 MCA slots (16/32 bit) Interfaces (onboard): Mouse, Keyboard 1x Serial 1 x Parallel Floppy (1.44M), allows attachment of up to 2 drives VGA ESDI hard disk (DBA interface) Add-on cards: SKnet Junior MC2 Ethernet Adapter IBM 8514/A Video Adapter IBM Dual Serial Adapter Operating System(s): Linux 2.0 (Slackware-based) Useful Links: Original Announcement from IBM Details about main board Reference Diskette Adding a Second Drive to a Model 50/70

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