About the differences between the TTL families
General characteristics of the TTL families (HC, HCT*, LS*...)
First there was normal TTL (Transistor Transistor Logic).
There were also things like DTL (Diode Transistor Logic).
TTL became very popular, very soon, although CMOS*
was also already used by many other people.
CMOS* and TTL were quite different in handling and
CMOS* was 3V to 15V. TTL was 4.75V to 5.25V.
CMOS* used much less energy because it was high-impedance.
In CMOS* every connection between the power lines, always
has two transistors in it's path of which one is always
Comment by Ian Cox of the UK:
CMOS* doesn't have NPN & PNP transistors but CMOS* N-channel and P-Channel FETS and it is actually in CMOS* that BOTH devices turn on and short out the supply for micro-moment during switching. I am not aware of this phonomenom in TTL logic (even old stuff).
CMOS* was much more sensitive to static electricity.
After a while LS* (Low Power Schottky) TTL was
invented, probably by Schottky. It used much less energy
and was as fast as normal TTL.
Everybody started using LS*. (Except for the fast versions:
S and FACT and FAST.)
These chips were for example used for a single critical
chip on a board, like a (primitive) MMU* which would otherwise
slow down the memory access to much.
TTL with it's higher power usage, had always been
faster than CMOS*. Therefore CMOS* was more used
for analogue and TTL for digital circuits, but after
some time, they invented HC (High speed CMOS*) and
made (High speed CMOS*, TTL-compatible) with it.
This means, that internally a HCT* chip is completely
High speed CMOS* and at the in and outputs are converters,
that convert the levels to TTL.
Comment by Ian Cox of the UK:
You advise the use of HC, but AC (Advanced CMOS*) or ACT (same again but it's input levels have been doctored to be compatible with old-fashioned TTL) has higher current drive capability (24mA!) and is generally faster still!
HCT* is mostly compatible with TTL, except:
- You should never leave an input open, because
the circuit can start to oscillate, which costs a lot
of energy and can disturb the working of the rest
of the circuit.
- You can't use the 74HCT04 as an oscillator (with a
crystal), as was often done.
But they designed a special version of the 74HCT04
for this: The 74HCU04!
- Normally use HCT*.
- If you want faster chips, use FACT/FAST or whatever.
- You can replace TTL-chips on old boards
generally with HCT*, unless it's an 7404 near
a crystal or the designer of the board relies
on the (slow) speed of certain components,
which he should never have done!
What is the difference between the 54 and 74 family?
The 54 family is meant for military purposes.
This means it is guaranteed over a larger temperature range and
is more expensive. But civilians may also buy it...
(When you want to make something that can also be used in cold
or hot climates for example.)
Is the power supply of CMOS* only 5V?
Yes, but real CMOS* generally can work over a great range of voltages.
In earlier days, chips needed all kinds of weird voltages. Now they
generate their own (low current) voltage internally by way of a charge pump,
Why would you mix TTL and CMOS* devices?
In earlier days not all functions were available in both TTL and CMOS*.
Now you can get most of the CMOS* 4000 series also in TTL. Type numbers are
Why did TTL-to-CMOS and CMOS-to-TTL require interfacing devices
TTL worked at 0 to 5 volt and CMOS* allowed/needed all kinds of strange voltages,
but was less critical about these voltages. But there are a lot of different
ways that CMOS* can be used to implement designs!
Currently even 'TTL' is done in CMOS*. HCT* means 'High speed CMOS* TTL compatible'.
What is ECL*?
It means Emittor Coupled Logic and it is extensively used in high speed digital data handling systems. Some NASA sites use ECL* to handle baseband data up to 300mbps. TTL just won't perform at that data rate.
J.R. "Zeke" Walton from NASA.
What is BiCMOS?
Bipolar CMOS*? Bipolar is generally very fast. But lately CMOS*
is also very fast...
I assume you know that CMOS* always has two transistors (an NPN
and PNP one) between any connection between the 0 and 5 volt power
supplies of which always one is not conducting. Early TTL
had very short moments in which the single transistor
switched and shorted the 0 and 5 volt which used a lot of
energy. DRAM (which had much more gates than TTL became so
hot that they warned you never to check with your finger
if they were getting hot... It was safer to moisten your
finger first so the temperature would stay below 100 degrees C.
CMOS* has been 'round for about as long as TTL was always
much more careful with energy usage, but it's much harder
to produce since it requires those PNP and NPN transistors
on the same chip which is a very complicated process and CMOS*
used to be very slow. The current VLSI however is so dense that
when it would have to be done in TTL that it would burn up
immediately because of heat problems. That's why CMOS* had to
be gotten under control and all VLSI is done in CMOS* by now.
They even had to lower the power voltages to keep the heat
GaAs was also considered or even used for very fast chips.
You probably also know that IBM had a couple of water
cooled mainframe computers in a period that their processors
couldn't be cooled well enough with air.
What to use in the daily practice?
Just use HCT* (and NMOS* if must be) components and other
TTL-like stuff unless you know why you would want to use
anything else... And try to find MCU's with as many
peripherals already integrated to save on part costs,
board space, CAD*, debug and programming time and increase
product reliability etc.
Additional questionFrom: Jaap van Ganswijk <firstname.lastname@example.org>
Subject: Re: Two Queries
At 11:54 19990217 -0800, Declan Moriarty wrote:
> I have one out of two queries on topic...above average perhaps ;-)
>1. Can anyone point me to a reference that tells me the difference
>between all the 74xx serieslogicfamilies? I am finding it difficult
>to get some 7400 series chips locally, like today I had problems
>with the 74LS01. I need to know could I shove in a 74ALS01, or 74F01,
>or 74L01. What is the difference between 74HC and 74HCT*...that sort of
I have written a page about it:
The generations of the most economical types were:
These are basically compatible but every generation is faster
than the last and uses much less energy. HCT* is CMOS*, so
you need to connect all the inputs which was not needed with
parts of the other generations. Also the 74ls04 that was often
used in an oscillator circuit couldn't be replaced by the
74HCT04, but they produce a special version, the 74HCU04
that can be used in this special way.
All other types of 74 chips were faster or used less energy
or whatever. They would have been more expensive, but if
that is no problem I don't see a reason not to use them as
Beware that the 74HC's may be real CMOS* and not TTLcompatible as the HCT* (=High Speed CMOS* TTL compatible).HCT* is CMOS* that has been made to act as TTL, so 0 and5Vpowerand probably levels of 0.8 and 2.7 V for low and
When a design is really critical I'd check the datasheets, but
I'd even do that with HCT*...
ExampleSince a lot of people seem to have trouble choosing, here a morepractical example. Suppose you would like to build a 6809 Unix computer.This would have to involve a simpleMMU(Memory Management Unit) which
dynamically translates the upper 4 address lines of the 16 logical address
to say 8 address lines to form a 20 bit physical address. The translation is
called dynamical since it has to be done at every read/write to memory.
This way the OS can assign every 4K physical page to every 4K logical
page of any task as it chooses.
The 6809 is a traditional processor which can't add wait states in his read
and writes, so the time from address available to data read is fixed and
limited. Normally there is enough time to select the correct chip from the
address given en produce the data on the data lines, but with the added
time required by the address translation mechanism the complete design
can't be done in HCT*. The translation mechanism consisting of two 16*4 bit
chips has to be done in a faster technology like 74S. Most of the rest
can be done in HCT*. Just some components in the timing's most critical
path will need to be done in faster technology. This only doubles (?) the
cost of a few components, but only ups the cost of the total design a
few procents considering the total number of chips involved.
By the way, we have built such a 6809 Unix computer around 1984 and useda couple of them for many years since then, both for database, accountingand embedded software writing. We never built a successor both because itwas hard to find a good 16/32 bits processor and because the 80286 wasalready then a viable similarly functional system withMMUon-board and
could run Xenix quite well. With the arrival of the 80386 there was
absolutely no incentive to build our own computers anymore. Motherboards
were getting cheaper and cheaper. Xenix was still expensive though. Linux
changed this of course. MS Windows also became more and more a serious
|74||Standard TTL||This was the original series. Was superseded by 74LS* and later 74HCT* for general usage.|
|74L||Lowpowerand lower speed.||Probably for portable applications.|
|74LS*||LowpowerSchottky TTL I/O.||Uses Schottky barrier diodes (from memory between the base and collector) to prevent the transistors saturating, hence improving speed when they turn off. Probably about as fast as standard TTL.|
|74ALS||Advanced LS* TTL||Faster than LS* and higher outputcurrent.|
|74F||Fast TTL I/O.||Uses lots ofcurrentto achieve high speed. Probably not that fast any more.|
|74C||CMOS*||Uses CMOS* transistors and hence has switching levels set at half supply unlike all of the above. These can usually be run of supplies from about 3 to 15V.|
|74AC||High speed TTL I/O|
|74ACT||High speed TTL/CMOS* Input CMOS* Output|
|74FC||High speed TTL/CMOS* Input CMOS* Output|
|74HC||High speed CMOS*||Faster using CMOS* transistors, half supply switching. 5v only.|
|74HCT*||High speed CMOS* with TTL switching levels||Uses CMOS* but designed toswitchat TTL levels (ie low = <0.6V, high = >2V)|
|74AHC||Advanced HC?||Faster than HC?|
|74AHCT||Advanced HCT*?||Faster than AHCT?|
More subtle differencesBy Andrew Ingraham
1. Can anyone point me to a reference that tells me the differencebetween all the 74xx serieslogicfamilies? I am finding it difficult
to get some 7400 series chips locally, like today I had problems
with the 74LS01. I need to know could I shove in a 74ALS01, or 74F01,
or 74L01. What is the difference between 74HC and 74HCT*...that sort of
There are many subtle differences that might come into play when you go
between families. Depends on your circuits.
In my opinion, the major differences from the user's perspective, are
speed, and input thresholds. Some CMOS* families use optimized "CMOS*"
levels while others use "TTL" or perhaps "LVTTL" levels. TTL's input
threshold is around 1.4V, versus 2.5V (Vdd*/2) for 5V-CMOS. TTL accepts
2.0V as "high"; CMOS* would call this marginally "low" and needs a much
higher voltage to be considered "high".
But when you come down to using them in your circuits, you also need to think about things like:
Inputcurrent, in both high and low states. If you substitute a 74HCT* with a 74F, thecurrent(i.e., loading) is a lot higher.Output drive capability.Output drive symmetry. Is it suitable for driving longbuswires, or will pulsewidths suffer?Signal integrity. Faster gates make faster output edges. 74F is fast, but watch it if you need to drive more than a few inches (depending on what it drives).Powerdissipation, if you have a lot of them.Clamping at inputs, I/Os, and tri-state outputs. If you use a 3.3V family, will it clamp above 3.3V, or is it 5V-tolerant? You can get both.Can unused inputs float? Some are OK with that, others aren't.Noise immunity. Faster gates will see noise glitches on their inputs that slower gates miss.
Get as many data books as you can and study them until you familiarize
yourself with all these differences. What may be an acceptable
substitute in one case, may be a flop in another.
See alsoNS's "High-PerformanceLogicSelection Guide".
TI's TTL databook. It gives a guite good overview of these logic-IC families, with properties, differences, etc...
Difference between 54 and 74 family
By Greg Smith
The specs for 54xx usually show them as being slower than 74xx,
although in fact this is probably just a derating for the extended temp range.
By Greg Smith
(Read this in the voice of Grandpa Simpson:)
Back in the 70's, you had 7400 and 74S00. Maybe you were just getting 74LS00.
If you wanted things to run at, say, 300 MHz, you could do it
with ECL*. ECL* logic uses a -5.2 V supply, and switches above and below
a certain threshold voltage, I think it's -1.2V. A lot of the devices had
+ and - outputs for the same function, or differential inputs. You could apply
+ and - ECL* outputs to a twisted pair cable, run it a few feet to a different
board, and apply the cable to a differential ECL* input, and it would
work at very high speeds. Since it switches current from one side to the
other, rather than turning it on and off, and since the voltage swings are
very small, ECL* had far less noise problems than TTL and would run at
high speeds on wire-wrapped boards. You needed to use terminating resistors
on every signal, though. Lots of power.
I encountered stuff like this inside a 70's era Control Data disk drive.
This thing had a 60 MB removable pack about 12" in dia and 6" thick. The
entire unit was about the size of a modern office photocopier, and
weighed more. The backplane was connected with wire-wrap.
The termination resistors in the interface cable used more
power than an entire modern HDD.
ECL* is not really used any more.
74H00 - higher power, faster than 7400
74L00 - low power, slower than 7400.
These were used before 74S and 74LS*, and were a direct power/speed tradeoff.
They were already obsolete in the late 70's when I started tinkering
with this stuff.
By Allan Warrington
Regarding the comment about ECL* not being much use for anything.
ECL* was quite important in the 1970s, 1980s and early 1990s.
It was used for very high speed circuits.
Early supercomputers e.g. Cray's were made with it.
However, it was very power hungry.
I think that it isn't much used nowadays.
If you try to use any old ECL* chips, the logic levels
are typically around 0.9V below VCC for logic high and 1.8V below supply for logic low.
Supply is generally 0V and -5V, rather than 5V and 0V.
From: Jaap van Ganswijk <email@example.com>
To: Silver Timothy (eeb2_99) <firstname.lastname@example.org>
Subject: Re: LS* TTL logic family
On 19991014 eeb2_99 wrote:
I am a student at portsmouth university and I am having trouble
It means transmission in this case and the propagation time or
propagation delay is the time in which a signal travels through
a gate. It's typically 4 ns for LS* TTL, I think. A simple NAND*
consists of a single gate. An AND* of two gates and for example
a 74LS138 has usually several gates from input to output so it
takes say 6 times 4 ns.
understanding this topic. Could you possibly enlighten me about;
"Typical gate propagation delays for the LS* TTL logic family"?
In serious PCB-designs you need to calculate how long it takes
for signals to go from serious chip A to serious chip B through all
the intermediate TTL-chips. If the signal takes too long the
circuit may start to behave badly (at first at higher temperatures
First you try to calculate everything exactly and then at an extra
proof you can put the finished product in an oven and heat it up
to check if it still works. This will give a clear indication of how
reliable it is.
Some more info ...
TTL normally uses only NPN transistors (although newer derivatives
As a result, TTL can sink much better than it can source, whereas
CMOS* can do both fairly well and has "rail-to-rail" output voltage.
Joe da Silva
Z-state (high impedance)
On 20010508 Himanshu Rawal wrote:
Can you tell me the meaning of the
term z-state(High impedance) for a digital
circuit and how do all the logic families
A digital output is internally usually connected
with one transistor to the 5V and one transistor
to the 0V. Only one of those is normally
conducting electrical current at a certain moment
pulling the output pin either to the 0V or the
5V. When none of the two transistors is
conducting, the output is in the Z-state (high impedance state).
The different logic families use different technology
to implement the transistors, but the principle is
probably the same.
Buy the way early chips were made in a technique
whereby both of the transistors would conduct
current for a short time during transitions.
This made them actually short the 5V to the 0V
for a very short time. Therefore these parts
would use a lot of energy and become very hot.
DRAM from around the time that about 16kbit per
chip was producable could easily burn your finger
so feeling the chip to debug the hardware was not
and advisable idea. You should at least wet your
finger first. If you like to repair old computers
like the TRS-80, Apple2, Commodore PET, BBC etc.
it's wise to remember this...
At 2001-07-31 10:46 -0500, R Rodd wrote:
In the TTT, CMOS* page (ttl.htm) you mention not leaving CMOS* inputs floating,
but should they be tied to ground or Vcc*, or doesn't it matter which?
Specifically at this time I'm looking at a 74HC14 hex inverter,
but would also like to know if there's a rule of thumb to follow
i.e. drive inputs such that the output is always high, or low,
or either of the two is OK.
It's not easy to determine what the optimal solution is.
The problem with CMOS* inputs is that they are very high impedance and may
pick up signals from the surrounding environment and the air and may start
to oscillate. This may not only cause highly increased power usage but
also other undesired effects.
So as long as you tie the inputs to a signal with well defined value at
each point of time, like a neighboring data pin, either in- or output or 0V or 5V
this problem will be prevented. For some technologies it seems to
be recommended to connect the 5V via a relatively low-R resistor
(4k7 for example). Power inputs of a chip can normally withstand
a power surge, but inputs may not.
What the optimal solution is depends on all kinds of factors:
Does the technology draw more current with a low or high input?
When connecting the input to a neighboring pin, you'll have to consider
the load on the given signal. But all this is in most cases not very important.
It's only relevant when for example battery life must optimal or signal
speed is crucial. In practical cases I'd just solder it to the 0V or 5V if
any is available on the next pin, or in case of a two-input (N)AND* or
(N)OR* which is just used as an invertor, I'd solder both inputs together
or otherwise it depends on which signals are available closeby and if they
can drive an extra load (which they usually can) and if their reaction
speed isn't crucial for the system (which it usally isn't).
In case of your hex invertor you could consider feeding certain signals
to two invertors parallel to each other and also tying the outputs together.
It will double the load on the input signal, but will also double the drive
of the output signal. It might however increase the power usage during
the moment of switching when the two gates should have significantly
different switching times, but that is of course very unlikely. ;-)
||even more about this subject