Alt-BEAM Archive
Message #01912
To: Benjamin Edward Hitchcock beh01@uow.edu.au, beam beam@corp.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Wed, 31 Mar 1999 07:19:16 -0800
Subject: [alt-beam] Re: 74HC240
Hello Ben
Tri-state only refers to the "floating" or high impedance state of the
240 output. The inputs are NOT designed to sense 3 different logic
states. ie zero, one and floating. Always tie unused inputs to 0V or +V
or else your chip may get quite warm!
Tristate outputs can be connected together on a common bus with only one
output active (ie high or low) at a time. A bus of this type often has
termination resistors which pull up the bus when all outputs on the bus
are tri-stated. At 5V supply voltage, if you allow all 74HC240 inputs to
float, the chip could draw as much as 150 mA of supply current! Just
try tying all HC240 inputs together and to the wiper of a pot with the
pot connected across the 5V supply. Then adjust the pot for maximum
supply current through the chip and stand back. The same thing can
happen if you let the HC240 inputs "float".
This also means that quasi linear circuits like the microcore, bicore,
and other Nv neuron circuits draw substantial pulses of supply current
when the voltage at the input of the 74HC240 or 74HC14 enters the linear
region. Fortunately, the voltage is in the linear region for only a
short time but the supply current pulses can cause circuit to misbehave
(saturation etc) unless you provide adequate bypass caps on the supply
pins of the logic chips.
Long time constant Nu circuits like the PNC will draw high supply
current for a longer duration. Solar engines, which are in effect mono
Nu cores, can draw so much current when entering the linear operating
region (close to the trigger point) that the solar cell cannot supply
enough current and the charging of the capacitor stalls preventing the
SE from firing.
This high current also makes it difficult to design a 5V solar engine
using logic gates. The problem is much less severe at 2V supply
voltage, and this "linear region" current drops down to a few hundred
uA. This is one reason why the Steven Bolt's Sun Eaters circuits must be
operated at 2-3V. It is also the reason why a 5uA supply current opamp
designed to operate in the linear region is such a good choice for a
solar engine (nuff said).
enjoy
wilf
Benjamin Edward Hitchcock wrote:
>
> The 74HC240 is listed as a tri-state line driver.
> What does tri-state mean? High, low, and floating?
>
> Would it differentiate between two voltages that were very close to half
> the supply voltage like a normal 74HC14 chip does?
>
> What I mean is, what outputs are possible for values of input very close
> to half supply voltage.
>
> Thanks for any help,
> Ben
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