Are bus switches inherently bidirectional?

[poofjunior]

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!

 

[JimB]

are these bus switches bidirectional?

Yes.

If so, how can I tell?

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

JimB

 

[Tony Stewart]

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

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.

 

[poofjunior]

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?

 

[Tony Stewart]

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?

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

 

[JimB]

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.

Further on in the text, it also states:

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

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

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

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

 

[Tony Stewart]

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

 

[poofjunior]

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