Alt-BEAM Archive

Message #04376



To: beam@sgiblab.sgi.com, Bumper314@aol.com
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Wed, 09 Jun 1999 21:13:29 -0700
Subject: [alt-beam] Re: SE questions not found in the FAQ.


Bumper314@aol.com wrote:

> ... When one row of inverters is enabled, do you have to use them all? I
> tried to make his power smart head, but the chip just ended up burning
> my finger and the motor never moved =).

You don't have to use them all, Steve, but the Motorola handbook
suggests you tie all the unused INPUTS either to ground or to Vcc.
Unused OUTPUTS are left unconnected. The reason given is, if you don't
do this the gates may switch on and off very rapidly, consuming power
and producing heat.

However, I don't think the kind of heat they're talking about will lead
to burnt fingers. From practical experience (i.e. a couple of smoked
chips), make sure your ground and Vcc are connected to the proper pins
(Pin 10 = GND, Pin 20 = Vcc for the 74xx240), and make sure you haven't
accidentically connected an OUTPUT to either Vcc or ground.

Also, be sure when you identify the pins, you're looking at the same
side of the chip as your diagram. Some people show a free-form layout
with the pins pointing up; data sheets show the chip pointing down.

Motorola's website has all the datasheets on-line in PDF format.
Everything you wanted to know about a chip is there (plus a lot you
probably didn't want to know). Their site has been undergoing some heavy
revision, but I was recently able to select any logic chip I was
interested in off this page:

http://www.mot-sps.com/books/current.html

There's a list of library collections on this page, and you probably
want the link to either "Fast and LS TTL Data" (LS, ALS chips) or
"High-Speed CMOS Data" (HC, HCT chips).

Regards,
Bruce

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