Re: Tri State Devices

Abstract Dissonance <Abstract.Dissonance.hotmail.com> wrote:
Is a Tri State "driver"(I think thats what its called) basicaly a device
which lets you set either Low Z or High Z?

something like

C
|
|------|
A ---| |--- B
|------|


Where, say, if C is high then A and B see a high Z(open circuit) and if C is
low then it acts like a low Z? (a short)

or maybe they are only unidirectional?

They are unidirectional (A->B) in your case, and digital only. They
shouldn't be thought of as a switch as they aren't. It's a digital buffer
whose output drive can be shut off.

But basicaly it is like a controled switch?

I think thats a bad description, since a pass-gate is much more similar to
a "controlled switch".

The reason I ask cause in a circuit they have something like



D2 >------------+
|
|
|------|
D0 >---------| |---+----- Data
|------| |
|
ACK >-------------------+

Where the device is 74LS244.

The schematic you drew is essentiall a bidirectional I/O. "ACK" sees
whatever is on the Data line. If D2 is asserted the tristatable buffer
asserts D0 onto Data. The theory here being that if you want to read, you
shut off D2, and Ack will have valid data being asserted from off the
schematic. If you want to write a value, D2 is asserted and D0 is
buffered through to Data. Ack will still be a valid read, but it will
simply read back D0.

One of the more common applications for this is on bidirectional busses.
For instance on the ISA bus, the CPU may want to pass data to the
peripheral boards by asserting D2, and then it may want to allow the
peripheral boards to pass data back to it over the same lines by
de-asserting.

.

Relevant Pages

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