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From: stevem@specialix.co.uk (Steven Murray)
Newsgroups: sci.electronics
Subject: Re: Distinguishing between logic state and tri state
Message-ID: <1991Jun10.124633.24692@specialix.co.uk>
Date: 10 Jun 91 12:46:33 GMT
References: <1991Jun3.193519.19595@uokmax.ecn.uoknor.edu> <1991Jun4.222654.11849@milton.u.washington.edu>
Organization: Specialix International, London
Lines: 45

whit@milton.u.washington.edu (John Whitmore) writes:

>In article <1991Jun3.193519.19595@uokmax.ecn.uoknor.edu> sashahi@uokmax.ecn.uoknor.edu (Sandeep Shahi) writes:

>>   Can anybody suggest a circuit which can separate a logic(0 or 1)
>>from a tri state.
>>		The output of such a circuit is to be read by the host
>>for further action

>	The obvious answer is to test the impedance of the pin; a 
>220 pF capacitor and 2.2K ohm resistor in series from a square wave
>generator

[...stuff deleted]

>	In the old days, one simply had a termination resistor that
>pulled the output into the TTL forbidden region (about 1.5V); no
>logic LOW or logic HIGH gate would ever spend long at that voltage,
>so a simple gate comparator would detect tri-state.  Alas, the
>range of pullup resistor values nowadays is large, and the behavior
>of some CMOS circuits with the inputs held in that range is terrible
>(it can destroy the chip).  Unless you know the value of pullup
>resistors on the tri-state lines, or unless you know there's no
>CMOS that could be damaged, that technique is not reliable.

>	John Whitmore

Pullup resistors are also going to load the circuit you are suggesting
here.  The circuit you are suggesting can not give you a dynamic indication
of a circuits tri-state condition, as pulling to a mid-supply (e.g 2v val)
can.  CMOS circuits are very unlikely to be damaged by a mid-supply output!
If you are talking about a CMOS input the only time you get really excessive
current draw from a mid-supply CMOS input is when you have the chip on a
low Vcc (about 3v) - in any case values can be picked to ensure the 
'tristate voltage' is away from the transition point of the particular
logic families being used.

-Steven Murray


-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
I must write out one thousand times "I will not leave myself logged in"
main{int a;for (a=0;a<1000;a++)printf("I must not leave myself logged in");}