Three state output devices

[engr_joni_ee]

I am wondering what are the three state devices. I am looking at the data sheets of SN74AVC8T245 and NC7SV125 but not able to understand the input/output relation in these three state devices. I will be thankful if someone explain it a little bit that how these devices actually works and that they do

 

[danadakk]

The output logic state can take on 2 "valid" states, and 1 state where the output
is unaffected by any device inputs :

A valid state where the output meets the Vol and Voh specs in datasheet for that logic
family. Different logic families can have different valid levels, TTL, COS, ECL, LVDS........

Thats when the output totem pole output transistors, the drive to a low and drive to a high,
are both off, Q3 and Q4 in this example. So pin V is only affected by leakage and thats
unpredictable. This is tristate logic and allows one to connect multiple devices to the same
buss, share the buss, one at a time when enabled to control the buss.

CMOS looks like :

Tri-state here is when both PMOS and NMOS transistors off.

Some designs will use R pullup or pulldown R's so that when nothing
is driving the buss the R's will force a logic 0 or 1 as a default on the
buss. Thats desired so that any inputs, in case of CMOS logic, do not
float and draw power in their input buffers. But any device that takes
control of buss can drive the R's to desired state.

https://www.ti.com/lit/an/scba004e/scba004e.pdf?ts=1672320881898&ref_url=https%253A%252F%252Fduckduckgo.com%252F

Regards, Dana.

 

[KlausST]

Hi,

three states:
* HIGH (driven)
* LOW (driven)
* not driven = High-Z = floating

ENABLE input decides whether the output is "driven" or left "floating"

Klaus

 

[FvM]

74xx125 is an unidirectional tri-state buffer, 74xx245 is a bidirectional tri-state buffer, transmitting data in both directions depending on direction control signals.

 

[dick_freebird]

You may also see "tristate with bus hold" where the main
drivers can go hi-Z, but a smaller cross coupled inverter
keeps the output voltage where it sat until somebody's
attached driver turns on and overrides it, to then hold at
new position. More of a recent thing, never saw it in the
bipolar families but do see it in more modern bus transceivers
(and ASICs) on occasion. Complements the pull up, pull down
with a "stay put" when-tristated option.

 

[engr_joni_ee]

Just one more question regarding SN74AVC8T245 and NC7SV125.
The input/output current, can we call them sink/source respectively ? as the input is current sink and the output is sourcing the current ?

I am also looking for the current rating per per channel. Is it +/- 50 mA for SN74AVC8T245 and +/- 24 mA for NC7SV125 ?

If the controller deliver less current then such buffers will make the output current larger to drive the load, right ?

 

[danadakk]

SN74AVC8T245

The above stipulates a single output can source of sink max of 50 mA. But that total
port current is limited to 100 mA. And in this condition logic levels are not in spec.

The outputs source or sink current depending on load and logic level conditions. The inputs
just small currents, see spec sheet.

If the controller deliver less current then such buffers will make the output current larger to drive the load, right ?

I assume your controller is not driving the buss same time as buffer, so question not appropriate.

I will let your review the other datasheet for limits.


Regards, Dana.

 

[barry]

Just one more question regarding SN74AVC8T245 and NC7SV125.
The input/output current, can we call them sink/source respectively ? as the input is current sink and the output is sourcing the current ?

I am also looking for the current rating per per channel. Is it +/- 50 mA for SN74AVC8T245 and +/- 24 mA for NC7SV125 ?

If the controller deliver less current then such buffers will make the output current larger to drive the load, right ?

You're getting your bloomers in a bunch. It's not that complicated.

1) Device will supply the NECESSARY current to the load, UP TO the rated maximum. These are voltage output devices, not current output.
2) When an output is high it will source current; output low will sink current.
3) The inputs don't source or sink very much current; think of the input as a high impedance.