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building a logic probe

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samcheetah

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i want to build a logic probe which will indicate a high logic level, a low logic level and a tristate. this probe should be designed using only logic gates (no 555 timers, PLDs or PICs etc). i have tried to make a few designs but im not sure about them. i also want to make an enclosure for it. so plz suggest a good enclosure for it.
 
thanx ante for your reply. im sorry i didnt mention that i want to build the probe to test TTL circuits. the circuit u gave tests CMOS circuits. i need a probe that will only test a HIGH, a LOW and a TRISTATE for TTL circuits.

thanx for the help in advance
 
Heya Cheetah,

I think I have what you're looking for, only I have the schematic at home right now. The probe shows High, Low, Tristate (oscillation) VIA a 7 segment display. The schematic uses 2 IC's and a couple resistors and other minor parts.

I'll post the schematic when I get home (a few hours or so).
 
Here you go. TTL only, unless you develop a switch to change the resistor for the display input, and use CMOS equivalent IC's.

The display will light an "H" for high, and a "L" for low. For the oscillation probing, both H and L will seem to be lit. This is due to Persistence of vision because the oscillations occur too rapidly for the human eye to distinguish between H and L displays. Thus, you'll see an "8" unless the oscillations are extremely low frequency.

I've seen some neat enclosures, which enclose the circuit in a tube which is about 1 inch I.D. or so. (PVC tubing is cheap!). The probe is fasted (Epoxy) through the center of a pvc tube cap, which fits onto one end of the PVC tube. The other end has a similar cap, with holes to allow the wires through. You can then paint the PVC tubing (Which looks nice if you do it right). After you finish, you'll have something that looks like an oversized marker, or an exploded view of a ball-point pen. The enclosure shouldn't cost more than a couple of dollars.
 

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  • Tri_state_logic_probe.GIF
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thanx Johnson. i must admit that the solution u gave is great. but there is one problem. i said that it should only have logic gates. but this one also has a timer (74121). i want a TTL compatible logic probe that will indicate a HIGH, a LOW and a TRISTATE (i.e niether high nor low) by using logic gates only.

thanx for all the help!!!!
 
thanx Johnson. i must admit that the solution u gave is great. but there is one problem. i said that it should only have logic gates. but this one also has a timer (74121). i want a TTL compatible logic probe that will indicate a HIGH, a LOW and a TRISTATE (i.e niether high nor low) by using logic gates only.

thanx for all the help!!!!

AHHHHHH horse pucky!!! I forgot about the monostable multivibrator. Sorry about that. I don't have any ideas with using only logic IC's.

Good luck! :D
 
Johnson777717 said:
AHHHHHH horse pucky!!! I forgot about the monostable multivibrator. Sorry about that. I don't have any ideas with using only logic IC's.

You can build a monostable with a couple of gates, a capacitor, and a resistor or two - but in any case, it's still not detecting tri-state, only pulses.

The spec required doesn't really make much sense, it asks for TTL compatibility, but then wants to detect tri-state - which isn't a TTL compatible state?.

Would this be another homework question?.
 
You can build a monostable with a couple of gates, a capacitor, and a resistor or two - but in any case, it's still not detecting tri-state, only pulses.

I was just thinking the same thing. Wouldn't you be building a discrete monstable using logic gates etc?

Can we define Tri-state? I'm thinking we all have different definitions. I interpret tri-state as being oscillations (High, flat, low--low flat high--high, flat low etc etc.). Can someone set me straight?
 
Johnson777717 said:
Can we define Tri-state? I'm thinking we all have different definitions. I interpret tri-state as being oscillations (High, flat, low--low flat high--high, flat low etc etc.). Can someone set me straight?

As I understand it (from tri-state buffers), if you set the pin as 'tri-state' it is effectively disconnected - so the pin can be affected by other connections to it, but the tri-state one doesn't interfere with it in any way.

As far as PIC programming goes, setting an output pin to be an input effectively 'tri-states' it.

I wonder if whoever set the question thought of it as an oscillation?.
 
i want a TTL compatible logic probe that will indicate a HIGH, a LOW and a TRISTATE (i.e niether high nor low) by using logic gates only.

I'm confused. When you say "TRISTATE (i.e. neither high nor low" do you mean probing a point that has no voltage? Wouldn't this be accomplished by probing the point and finding that no readout occurs? Thus, no voltage is evident?

What is your definition of TRISTATE? Maybe we can help with this.

I'm under the impression, that TRISTATE refers to a probe that can measure:
First state : Lows
Second state: Highs
Third: oscillations

Thus, a TRISTATE (or three state) probe is accomplished. Any ideas?
 
Three-state logic, also calle tri-state logic ( a trademark of National semiconductor Corp, who invented it ) is a misleading name. its not digital logic with three voltage levels or HIGH-LOW and the LOW-HIGH type of thing. it's just ordinary logic, with a third output state: open circuit. the predecessor to TRISTATE was open collector logic.

The spec required doesn't really make much sense, it asks for TTL compatibility, but then wants to detect tri-state - which isn't a TTL compatible state?

TRISTATE isnt TTL compatible???????????? i dont think so

Would this be another homework question?.

yes it is
 
samcheetah said:
TRISTATE isnt TTL compatible???????????? i dont think so

I do! - TTL is either HIGH or LOW, a simple logic design, with the specifications well defined. Tri-state (which, from your description, is exactly what I suspected) is not either, it's a simple floating pin, and doesn't meeet TTL specifications.

I can think of a number of ways to check if a line is 'tri-state', but none of them could be done with TTL logic chips, simply because it's not a TTL level.
 
the 74LS125A is a Quad bus-buffer gate with three-state outputs. the 74LS240 is an Octal inverting three-state driver. the 74LS244 is an Octal non-inverting three-state driver. the 74LS374 is an Octal three-state positive-edge-triggered D flipflop.

all these ICs are TTL. and have a look at

**broken link removed**

and u say three-state (or better well known as TRISTATE) does not meet TTL specifications.

look i dont want to get into a tristate fight or something like that. i just want an idea from u guys for a circuit that will do what i want.
 
samcheetah said:
all these ICs are TTL. and have a look at

**broken link removed**

and u say three-state (or better well known as TRISTATE) does not meet TTL specifications.

It still doesn't, TTL can either be low (below a certain voltage) or high (above a certain voltage) - it's a binary logic system. There's a certain region between these two voltage where it's not a valid signal (not meeting TTL spec). But this isn't a 'tri-state' region, it's just not allowed in TTL.

So called 'tri-state' TTL chips simply switch the outputs completely off, not high or low (that depends entirely on the external circuitry) - it effectively removes the chip from the circuit.

I see no way to detect a chip set to tri-state with the limitations you've set, also in a circuit using tri-state chips, if one chip is set to tri-state another one is usually switched to take it's place (which is usually why it's done) - in which case there would be no way you could detect it.
 
So called 'tri-state' TTL chips simply switch the outputs completely off, not high or low (that depends entirely on the external circuitry) - it effectively removes the chip from the circuit.

this is the so called TRISTATE. this is what TRISTATE is. the output is completely off.

if one chip is set to tri-state another one is usually switched to take it's place (which is usually why it's done)

i agree

u know everything and still u say it isnt TTL logic.

in which case there would be no way you could detect it.

there has to be some way

plz help me
 
samcheetah said:
in which case there would be no way you could detect it.

there has to be some way

You could only detect it if another chip isn't holding the bus in a TTL state, other wise it would be either high or low, as the TTL spec requires.

Assuming it is in a tri-state mode, and no other chip has control of the line, two identical resistors one to +ve supply, and one to 0V would hold the line at 2.5V - which you could easily detect, but not with your limitation of only using TTL logic chips. The resistors (assuming the vlaues were chosen correctly) wouldn't affect the normal high/low operation of the probe, but when the probe is disconnected (or connected to a tri-stated line) the voltage would go to 2.5V.
 
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