26 Aug
2014
26 Aug
'14
5:09 p.m.
On 2014-Aug-26, at 3:27 PM, Eric Smith wrote:
I wrote:
(Sent this an hour earlier in response to the earlier response, but it hasn't shown up
yet, so replying here instead)
Did you explicitly want a reference from TI, or something explanatory from an
'authority' ?
The characteristics and design treatises at the beginning of Fairchild TTL databooks
address it in a little more detail.
(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
On Tue, Aug 26, 2014 at 2:46 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
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.
For "plain" TTL yes. One of the purported advantages of LS was not
needing to do that. I forgot about that detail when commenting on the
TI datasheet lossage.
Something about supply line spikes causing
breakdown of the input
transistor.
I've heard that explanation and I suppose it sounds plausible but I would like
to find an authoritative statement from TI (i.e., in a TI data sheet
or application
note).
Fairchild AN-363 "Designing with TTL" dated June 1984 mentions
standard TTL, LS, S, ALS, AS, L, and F (but omits H), and states
without reference to a specific family:
A good design rule is to tie unused inputs to a solid logic level.
Inputs are usually tied to V CC through a 1 k? to 5 k? resistor,
since tying them to ground means supplying the I IL current
instead of the I IH current. I IL is several orders of magnitude
greater than I IH . The resistor is recommended to protect the
input against V CC voltage surges and to protect the system
against the possibility of the input shorting directly to ground.
A single 1k resistor can handle up to 10 inputs.