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The model 50 was among the first four PS/2 machines IBM announced in 1987, and it is the only 286 desktop Microchannel machine that was ever made by IBM. The 286 CPU is clocked at 10 MHz, not fast compared to what competitors offered at the time, but this frequency gave the bus its standard operating frequency, and the bus itself was definitely a performance enhancement compared to the old ISA bus. The Model 50 is one of the few PS/2's where memory cards are not slower than on-board memory...
Actually, there is a difference between the the original Model 50 and the 50Z: After some time, customers started to quarrel about the 50's low performance and about its 20MB MFM hard drive. It 'featured' an average access time of 85 ms which was slow even then - the 'standard' 20 Mbyte Seagate ST-225 drive used by most Noname clones at that time had 65 ms, and drives with 28 ms were already available. IBM therefore modified the 50's memory interface from one to zero wait states (that's what the 'Z' stands for) and replaced the drive with a 60MB ESDI drive. These modifications, especially the faster and larger drive, turned the 50Z into a nice little machine for light DOS-based interactive applications like word processing.
Not only the bus was revolutionary, also the mechanical design was different from any PC before: This is Plug'n-Play applied to computer hardware. Except for power supply and main board, all components can be removed and inserted without tools. Screws have large heads so they can be turned by hand, and all drives are mounted on special rails to allow snap-in mounting. I remember a professor during my time at the university who brought a model 50 into the lecture-room one day, disassembled it and declared it as a good example about how things should be built! Admittedly, he handed the parts to one of his assistants and let him reassemble the whole thing ;-)
The space inside the 50 is well used, except some space around the hard drive; maybe even a 5,25-inch drive would fit?
Even the fan is simple to replace: pull up the two white plastic pins (a
special extraction tool is held available below the speaker), pull it out
and insert a new one! The way the hard drive is contacted may already be
called artistic: the ESDI controller has a special slot and the drive is
snapped in place. In fact, it is difficult to find any cable connection
in the machine...
To be honest, the ESDI controller cannot honestly be called a 'controller': The ESDI drive is of the 'integrated' type, i.e. the real controller is integrated into the drive's hardware and the drive's connector carries almost a full MCA bus. The controller only carries the BIOS and a few bus buffers. This controller is obviously not the adapter originally used in the 50Z - the 50Z's main BIOS already has support for ESDI drives, and the original 50Z adapter is an almost empty board (just a mechanical adapter). This variant with a BIOS extension was made as an upgrade option for the original Model 50 which doesn't have ESDI support in its main BIOS.
This machine doesn't have the original 286 CPU. I upgraded it with an SLC/50 CPU module made by Kingston that gives a nice performance boost and allows to run 32-bit software. Originally, this module was made for 286 machines with a PGA socket for the CPU. IBM however switched to the cheaper PLCC case with the 50Z, so I had to unsolder the CPU socket and replace it with a PGA type. One could still put the 286 back in place because it is possible to put a PLCC socket on top of a PGA socket. The module itself offers a slot for a 387SX coprocessor; you cannot keep a 287 coprocessor when upgrading. That's not as bad as it sounds since a FP emulation on a 50MHz CPU is almost as fast as an old 287.
The 50Z's main board is much smaller than the planar of the original 50: almost half of the case is free. IBM used higher-integrated chips and put parts of the electronics (VGA memory, floppy clock generator and PLL) onto small riser boards. The non-standard 512Kx9 30-pin-SIMMs were replaced with a single PS/2 SIMM. Other sizes than 1 and 2 MB however do not work, so you have to victimise a slot for a memory board :-(