Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!nprdc!malloy
From: malloy@nprdc.arpa (Sean Malloy)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: ISA boards, EISA bus
Message-ID: <9680@skinner.nprdc.arpa>
Date: 20 Sep 90 18:45:13 GMT
References: <1990Sep19.233544.16757@hayes.fai.alaska.edu> <1471@gold.GVG.TEK.COM>
Reply-To: malloy@nprdc.arpa (Sean Malloy)
Organization: Navy Personnel R&D Center, San Diego
Lines: 48

In article <1471@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Greg Ebert) writes:
>There are 2 tiers of connector 'fingers' on an EISA card. 
>
>	 \_______________________                      ____/
>				 |____________________|  <---- ISA fingers
>				   |________________|    <---- EISA fingers
>
>
>When you put an ISA card into an EISA slot, the physical width of the ISA bus
>connector area doesn't allow the card to reach the EISA signals.
>
>When you put an EISA card into an EISA slot, the card seats all they way down
>with ISA signals going to the ISA fingers, and EISA signals going to the
>EISA fingers.

Close, but no cigar; you've got the basic principle right -- the EISA
cards do go in deeper to hit a second row of contacts -- but you've
got the mechanism wrong. The EISA cards have the same length of edge
connectors (measuring the long way on the card), but the connector is
twice as deep. Where a 16-bit ISA card has one notch (between the
8-bit and 16-bit extension connectors), the EISA card has three; the
additional two only go halfway up the connector flange:

ISA:  |_________________________        __                   ___|
                               |________||___________________|


EISA: |_________________________        __                   ___|
                               |   __   ||          __       |
                               |___||___||__________||_______|

(The positions of the additional EISA cutouts may or may not be as
depicted here; I'm working from memory)

EISA sockets have flanges across the width of the socket halfway down;
when an EISA card is inserted, the flanges match the cutouts, and the
card can seat all the way down the socket. When an ISA card is
inserted, the flanges prevent the card from going any farther in than
is necessary for the card to connect to the first row of contacts,
which match the ISA bus contact layout.


                                               | "The three most dangerous
 Sean Malloy                                   | things in the world are a
 Navy Personnel Research & Development Center  | programmer with a soldering
 San Diego, CA 92152-6800                      | iron, a hardware type with a
 malloy@nprdc.navy.mil                         | program patch, and a user
                                               | with an idea."