Are bus switches inherently bidirectional?

Hey everyone,

I came across a 10-bit wide bus switch, the SN74CBTD3384, and I was wondering: are these bus switches bidirectional?
If so, how can I tell?

I've attached the schematic for each 5-port bus, controlled via an enable.

• https://www.ti.com/product/sn74cbtd3384 datasheet link

Thanks!

are these bus switches bidirectional?

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Yes.

If so, how can I tell?

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The switching element is a simple field effect transistor, whuch shows as a low resistance when the switch is ON.

JimB

JimB said:

Yes.
The switching element is a simple field effect transistor, which shows as a low resistance when the switch is ON.
JimB

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Incorrect. The simple FET switch is just a logical equivalent circuit is non inverting but is not the actual design, as a switch does no level shifting at all.

It is correct to say it is two (2) back to back tri-state buffers to make it bidirectional to shift between 3.3V and 5.0V for ports A & B, each being exclusively enabled or neither using tri-state.

Ah, thanks for the clarification, Tony. In that case, you're saying that the diagram that comes with the datasheet is merely a logically equivalent circuit and not the actual implementation?

In reality, the implementation is most-likely two back-to-back tristate buffers that do the level shifting and bidirectional communication?

poofjunior said:

Ah, thanks for the clarification, Tony. In that case, you're saying that the diagram that comes with the datasheet is merely a logically equivalent circuit and not the actual implementation?

In reality, the implementation is most-likely two back-to-back tristate buffers that do the level shifting and bidirectional communication?

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Yes! is will be SOME type of cascode level shifter. I can only guess which type, but not the example above.

That is very interesting Tony, but if we look in the various TI Application Notes associated with the SN74CBTD3384, we come across this document:
https://www.ti.com/lit/an/scda003b/scda003b.pdf
At the top of page 6, it clearly states:

The CBT bus switches consist of an N-channel MOSFET with its drain and source connected from input to output.

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Further on in the text, it also states:

The bidirectional nature of the switch is not sacrificed in this translation.

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So, going back to the OPs first post where he states:

are these bus switches bidirectional?
If so, how can I tell?

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I think that the quick answers are:
Yes.
Read the application notes.

I would also add the caveat that it would be a good idea to test that the device does what you really want before using it in a final design, especially if the application is not as a bus switch.

JimB

A switch has little or no offset, while apparently the level shifter does. Perhaps its a trade secret

Thanks for the link! I missed that line in the application note on the SN74CBTD3384, so I wasn't sure to begin with.