Bernie -
OK, You asked for it, so here goes -
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
ISA (8-bit - The ORIGINAL IBM PC and XT bus)
Single section, single level edge connector, 31 "pins" per side, 62 total.
8-bit data bus. 20-bit address bus, thus 1Meg maximum addressable memory.
Originally spec'd at 4.77MHz by IBM, the ISA specified a maximum of 8MHz.
----------------------------+ +---|
| UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU | |
\---------------------------------/ |
|
ISA (16-bit - The IBM AT and later)
Double section, single level edge connector, w/36 added pins, 18 per side.
16-bit data bus. 20-bit address bus - Maximum 16Meg addressable memory.
Originally, the IBM AT had a 6MHz bus speed. Later systems ran at 8MHz.
-----+ +-+ +---|
| UUUUUUUUUUUUUUUUUU | | UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU | |
\--------------------/ \---------------------------------/ |
|
EISA (32-bit - Extended Industry Standard Architecture. Not very popular.)
Double section, double level edge connector. Upper level was the same as 16-
bit ISA (AT), traces leading to lower level's pins were interdigitated
between the pins of the upper.
There were narrow slots cut in the lower level of the card edge. These slots
correspond to bars that blocked the lower half of the socket, and prevented
a standard ISA card edge from making contact with anything but the upper
level connections. You could safely use any ISA card, 8- or 16-bit, in an
EISA slot.
32-bit data bus. 32-bit address bus, 4Gig maximum addressable memory space.
Not sure, don't have the EISA spec handy, but I _THINK_ 8MHz max bus speed.
-----+ +-+ +---|
| HHHHHHHHHHHHHHHHHH | | HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH | |
| UUUUUUUUU UUUUUUUU | | UUUUUU UUUUUUUUUUUUU UUUUUUUUUU | |
\---------- ---------/ \------- ------------- -----------/ |
|
(An enlarged example of the interdigitated contacts)
|XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| | / Upper
|XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| | / Level
|XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| | \ Contacts
|XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| | \ (ISA)
|XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| H |XXXXX| |
__ __H__ __H__ __H__ __H__ __H__ |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
XX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |XXXXX| |
___________________________________________________________/
Since the EISA design required closer tolerances and an expensive socket,
not to mention a special chip-set on every I/O card to allow them to auto-
configure (a predecessor of Plug-n-Play), EISA cost more to implement.
Most EISA-bus motherboards ended up in network server systems, since their
moderately fast throughput and large number of bus slots (some had as many
as 14 slots) allowed them to handle greater numbers of disk controllers,
network interface cards (NICs), and multi-port serial interface cards.
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Hope this wasn't excessively large.
Dave
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