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# TTL, CMOS, EIA voltage ranges

Posted on 2003-10-30
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Last Modified: 2013-12-09
Hello people!
I need your help.

I have a test at my job comming soon and I might be asked about TTL, CMOS and EIA voltage ranges.
I have been google-ing for 4 days now, but I haven't found anything useful.

I know that the EIA voltages are +15 to +3V for 1 logic and -15 to -3V for 0 logic.
Also I know that for TTL for 0 logic the range is 0 to 0.8V.

PLEASE HELP ME! I know I have studied this in faculty, but I don't have the courses.(they are at my parents' home as I don't have to much storage space here)

PS.:As this is an urgent matter, I will award a lot of points....
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Question by:imnoguru
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5 Comments

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Accepted Solution

jhance earned 250 total points
ID: 9657008
1) TTL.  It's not STRICTLY defined what voltage levels are 0 and 1 for TTL since it's NOT a true standard, but they are generally accepted.  If you look at a datasheet for a TTL device you'll find 4 specs that are important here:

a) Voh - Voltage (output HIGH)
b) Vol - Voltage (output LOW)
c) Vih - Voltage (input HIGH)
d) Vil - Voltage (input LOW)

For TTL to work, you must have Voh > Vih and Vol < Vil.  Otherwise no guarantees can be made and you may have operational problems.  Typically, vendors spec their TTL parts like:

Voh (min) = 2.4V, Vol (max) = 0.4V
Vih (max) = 2.0V, Vil (min) = 0.8V

By using these numbers you get at least 0.4V of "margin" at either the "1" or "0" value.  Note that USUALLY, Voh is closer to 4.0V and Vol is closer to 0.2V for TTL so things may be better than "worst case".

Note that these number can vary depending on the TYPE of TTL you're talking about.  There are multiple families of TTL out there, "standard", "S", "LS", "F", "HCT", as well as others and these number can vary.  Some types of "TTL" are not fully compatible with others and you need to factor these things in when mixing families.

2) CMOS is more complicated since there are MANY types of CMOS, some "TTL" compatible, some not.  Some CMOS are 5V only, others are wide-voltage-range compatible.  With TTL compatible CMOS, the specs are similar to TTL but generally CMOS gets much closer to the "rails" (i.e. the POWER supply voltages) than TTL.  With wide-voltage-range CMOS (i.e. the 4000 series CMOS) the output voltages are usually referenced to the POWER supply voltage.  So Voh may be spec'd as Vcc - 0.2V.  Meaning it will produce the power supply voltage - 0.2V.

3) EIA is incorrect in this context.  EIA stands for Electronics Industry Association which is a STANDARDS MAKING ORGANIZATION.  There are MANY specs produced by EIA.  I assume  you are referring to EIA-232 (a.k.a. RS-232).  This is perhaps the most familiar EIA spec for most computer users.

The EIA-232 spec. SPECIFICALLY DEFINES the voltages and states for a STANDARD COMPLIANT interface. To know what these are, you read the spec.  But your ranges (1 = +3 to +15, 0 = -3 to -15) are typical.  Note that the STATE of 0 and 1 are NOT specifically defined and may be open to implementation specific interpretation.
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Expert Comment

ID: 9666492
>>>PS.:As this is an urgent matter

It seems very odd to me that you say, on one hand, that this matter is "urgent", while, on the other hand, you leave this question IDLE for more than TWO days now...
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LVL 1

Author Comment

ID: 9669466
the question was idled 'cause of lack of inet access during the weekend. The term urgent reffers to working days so, for me it was not idled not even one hour ;).

thanks for the answer, I managed to find a good book that specified the TTL and the CMOS voltage levels for every family. OTOH the remark about the states for RS232 not being explicitlly defined for one or the other voltage range is an valuable info, so I will give you the points, as you were the only one to answer
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LVL 32

Expert Comment

ID: 9669702
Well perhaps you'd be better off just aking the moderators to delete this question.  I'm not sure that I like having my thoughtful comment accepted merely because nobody else had anything to say.

I'll try to remember NOT to offer any comments to any of your future questions, OK?
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LVL 1

Author Comment

ID: 9669883
???? why are you so angry ????
the answer was good, in my opinion it was a B.

I said the only answer as in "I wish there were more answers from which I could get more information and choose the best from there". Just one good answer is ok, so you get the goods.

Don't look always on the bad side!

Best regards, 'cause ImNoGuru
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