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The Model 90 was introduced as a high-end desktop Microchannel machine, and it sometimes referred to as a 'desktop server'. In some way, it can be described as the desktop version of the Model 95, since it shares the concept of the CPU and central system logic on a separate card, the Processor Complex. In theory, you could even put a Pentium-based processor card into this machine, but IBM itself never offered the machine in this configuration: A Pentium-60 would drive the power supply in a well-equipped machine to its limits, and when the P90 complex was released, the 90 had long been replaced by the Model 77. The Model 90 also never got a new, redesigned 95xx planar like the 95, another indication of the MCA bus on the desktop? We will never know...
An uncommon feature for 85xx PS/2 systems is the second serial port. If you look into the MCA configuration, you will notice that the second serial port is shown as a separate 'virtual' card on an MCA slot that is not physically present. This shows that the second port is not part of the planar's core logic.
The on-board video logic is the equivalent of the IBM XGA Graphics Adapter, quite an effective upgrade from a VGA/8514 combination, but still limited to 43 Hz interlaced at 1024x768 pixels.
The 90 has quite a lot of space for devices. After the first generation(s) of PS/2 machines, it was the first desktop that offered space for a 5.25 inch drive (e.g. a floppy or a CD-ROM). Below the 5.25 bay and the floppy, there are two bays for 3.5 inch hard drives. 4 slots are available for extension, and all are full 32-bit slots (opposed to earlier systems like the Model 80). An extra fan is provided at the front, cooling both the add-on cards and the CPU board. An extra plastic piece directs the airflow towards the CPU on the CPU module, allowing even a 50 MHz 486DX to operate without an additional heatsink!
At about the same time, IBM introduced a new SCSI adapter that is equipped with cache memory. The standard amount is half a megabyte, but it can be extended to 2 megabytes by swapping the memory modules. Don't be tempted to add two industry-standard modules from your junkbox, they won't work! IBM used modules with a different CAS layout and additional presence detection. See here about how to modify industry-standard modules.
The Model 90 allows inserting up to 8 72-pin SIMM modules. Such an amount obviously didn't fit any more onto the planar itself, so IBM had to move the modules to raiser boards that are located next to the power supply. A plastic holder secures these raisers in their position. Usually, you have to fill the two raisers symmetrically with modules. The Type 2 complex used in this machine is an exception: you may fill up asymmetrically, and you may even leave one raiser out completely (like in this machine, since one of the raisers got lost over time).
The Type 2 processor complex was offered in two flavours: either with a 486SX25, or a 486DX2/50. In the SX case, a second socket is present that allows extension with a 487SX. After doing this, you may pull out the SX since it gets turned off - the 487SX is a 486DX with a slightly different pinout. Other manufacturers managed to provide a single socket that can take both SX and DX processors with either no or just a few jumpers, and so did IBM in later machines like the 77.
Since this board already contains a DX2, the socket for the 487SX is not present.