The Computer Destruction Lab
Epson XT-Class Computer Dissection
This is an XT-class 8-bit computer manufactured by Epson back in the 80's, and
donated by J. Garvin. You will get a feel for the big picture of computer design
(no pun intended). Though
many of the modern versions of the parts are smaller or already included
on the motherboard, they are all still there.
First, the whole computer from the back.
In the upper left, you can see the PC Speaker mounted on the side of the
Disc Drive Rack, containing a 51/4 Drive (top)
and an MFM Hard Drive (bottom). The three cards on the right are, in order,
a Drive Controller (with ribbon cables to the drives), a memory card (I
believe), and an I/O card for Parallel (printer) and serial (mouse or modem)
connections. In the lower left, you can see the power supply with the
massive transformer.
From the side, you can see more detail in the power supply.
The cylindrical black capacitors and transformer show. Notice also how
the cards took up the whole form factor of the computer, with braces
on the front.
The front of the computer shows what a user would have seen.
The on/off switch pushes a metal rod all the way back to the
power supply (see also the first image), physically cutting
or connecting the power. Also, note the MFM Hard Disc Drive's
green LED for activity, shown directly on the drive instead
of being elsewhere on the machine.
The expansion card daughterboard is shown here.
The cards are removed, and they connect with 8-bit
ISA (Indistry Standard Architecture) Slots. Note also the fan in
the power supply to keep everything cool.
The Disc Controller.
The Ribbon Cables still connect the controller to the MFM Hard Disc
Drive. The wide cable carries data, and the narrow one transmits
instructions. Note the pins for another narrow cable; two drives
could have been placed on this card, each with their own instruction
cable but sharing a data cable. The data cable is 34 pins, the same
as a floppy drive. The chip with the white sticker is an EPROM (Erasable
Programmagle Read-Only Memory) which could have been re-written if
that sticker were removed and ultraviolet light applied to the
glass beneath. Modern computers generally use SCSI or ATA/IDE based
drives insteead of the MFM shown here, and this card is often built
into the motherboard.
The I/O Card.
This card manages some of the Input and Output (I/O) in this case a single
25-pin connector on top, and a 9-pin connector on bottom. Though these
could both be serial connections, it's been my experience that most vendors
include parallel as one of the 25-pin connectors. The brown "H" chip is the
EPROM, the metallic chip on the bottom row 3rd from last is
probably the quartz clock chip. Modern ATX PC computers have all I/O built
in to the motherboard.
What I've been calling the memory card.
This card certainly has a lot of memory chips on the right, with room for more.
However, I'm not sure why there would be a connector on the back (left) of a
memory card that goes out of the computer. Near the middle is a button battery,
like most computers use to store settings from one boot to the next in their CMOS
(Complimentary Metal-Oxide Semiconductor) memory. In a blue bank most-of-the-way
to the left are 8 DIP (Dual Inline Package) switches. I don't know why either
of those parts would be on a memory board either. There are also resistors,
capacitors, connector pins, and jumper pins all over this card. It may be a
multifunction proprietary daughterboard for the Epson XT that takes some of the
load off the motherboard, and simply happens to fit in an 8-bit ISA slot.
These would all be on the motherboard of a modern system, and the memory
would be in a small DIMM (Dual Inline Memory Module) socketed directly into
the motherboard.
Here you see a bunch of the less interesting parts to look at.
On top is the (very magnetic) PC speaker, then a Seagate Hard Disc Drive
that I've been calling MFM. It may be RLL instead, I'm not sure how to tell the
difference. Under those is the Floppy Diskette Drive for 5 and
1/4 diskettes. Some could hold 360K, but newer ones
were double-sided and wrote double-desity, taking them to 1.2M. The Power
Supply is exposed here in all its glory: Capacitors, Inductors, Transformer
on the right, and cables to power the motherboard and Disc Drives.
Here, the raw motherboard.
In the upper-right, the expansion card daughterboard connector is basically an
8-bit ISA connector itself. To the right are two connectors, one of which I
believe is a parallel port and the other goes to a CGA monitor. Just to the
left of those back connectors is a socketed XT-class processor, either an 8088
or 8086 chip. The empty slot next to it would have held a math coprocessor if
one had made the xtra investment. In the upper center is a white bank of pins,
where the power supply connects. Just to the left of that is the motherboard's
EPROM, where it stores the BIOS (Basic Input Output System). On the far left is
a button that would have stuck out of the front of the computer, the reset switch.
Just below that is a bank of DIP switches which controlled the configuration of
things like Black and White versus Color, or 40 versus 80 columns. The pins in
the bottom center, I believe, were the on-board floppy drive controller, though
the smaller banks of pins make me wonder if it would have supported MFM natively
as well. The clock chip, in the lower right corner, probably ran under 2.5 MHz.
The same board from the other angle.
Here, removed from the last bit of case, is the motherboard. Visible in the
upper-right of this photo is the computer's keyboard plug, where a DIN-9 plug
keyboard could have plugged into the front (instead of the back, like many
others) of the computer.
The bottom of the motherboard
Shown for completeness. Notice the traces on the board that printed circuitboard
uses instead of wires for the chips and electrical components.
This ends the tour. Email me with comments or corrections.
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