26 Aug
2014
26 Aug
'14
4:08 p.m.
On 2014-Aug-26, at 1:46 PM, Tony Duell wrote:
This is addressed in, for example, the characteristics and design treatises at the
beginning of Fairchild TTL databooks.
(from the 1978 version):
"For a permanent high signal, unused inputs can be tied to Vcc.
A current limiting resistor, in the range of 1K to 5K, is recommended for emitter-type
inputs since these break down
at some unspecified voltage over 5.5V and power supply misadjustment or malfunction can
cause damage unless
current is limited.
.. diode-type LS-TTL inputs have breakdown voltages above 15V and thus protective
resistors are not normally required."
Sidetracking, it's interesting to note that most LS-TTL isn't 'really'
TTL, it's DTL with Schottky input diodes.
Some moron at TI has revised the SN5404/SN54LS04/SN7404/SN74LS04
datasheet to claim that all unused inputs must be held at Vcc or GND, which is
good practice, but to further cite their app note "Implications of
Is it? I thought that for plain TTL and (I think) LS it was good practice
to pull unused inputs up via a resistor and not link them straight to
Vcc. Something about supply line spikes causing breakdown of the input
transistor.