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DPRG: Logic Families

Subject: DPRG: Logic Families
From: MIKE MCCARTY -- 93789 JMCCARTY at DEVEL.USA.ALCATEL.COM
Date: Tue Aug 24 19:06:12 CDT 1999

[snip]

)> or somewhat less than 1/2 that required. For LSTTL, you could go as high
)> as about 3K for the pull down.

[snip]

)> I think you are still not out of the woods. If you intend to use LSTTL
)> parts, then you must *not* rely on an 8.2K pull down. That's too high.

)That is correct, we are replacing the 10K pull down with 2.2K SIPS. It
)seems to work. The voltage is measured at about .3V when not being
)read(SH = Hi), and about .4V when being read (SH = Lo).  This is about
)half way below the .8 V necessary for a low.

2.2K sounds about right to me. See my earlier comment above.

)I am currently choosing a transceiver like the '245 style.  I need to
)talk from one board to 10-12 boards on a 23" chassis.  Currently I am
)leaning towards a 74AHCT245.  Is there something better?  I am planning
)on running the backplane at 2 MHz or 1 MHz. The transceivers will be
)mounted as close to the connector on each board as possible.

I am not familiar with the AHCT line. But generally the "A" lines are
higher speed and higher current versions. 23" is not too far for
ordinary 245s, I'd think. I'm a little bit afraid of the "A" lines for
this type of application, especially at the low speeds you're talking
about. I bet the AHCT have something like 2ns rise times. Your bus
could ring like a struck bell. I'd use the slowest chips consistent
with the bus speed.

)I am probably going to have to terminate the ends on the motherboard
)because the lines are so long and have just started looking into that.
)The thing that is not clear from the books is whether both ends have to
)be terminated.  I would think so, but all the examples show only one.

At 2 MHz, I'd use slower chips, and put a 'scope on there and take a
gander. You likely only need to terminate the far end. Termination of
busses is an iffy thing in any case. Do you have insertable boards? If
so, then the termination impedance depends on what boards are inserted
between the master and the terminator.

Mike

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