Switch and Buffer

Hi,
I have to use a Bus Switch/Buffer for some uni-directional signals.
So under this circumstances, Bus Switch/Buffer should be the same for
me because both devices control the output using an OE pin as well.
But I believe there should be some differences between these 2
devices. Please let me know if you know the differences. What about if
my signals are bi-directional ?
TIA.

 

Wong,

The main difference between a bus switch and a buffer is that a bus
switch does not provide active current drive, whereas a buffer does.
A bus switch uses a pass transistor structure, so it is like a
conducting wire (with some finite ON resistance) when ON and an open
switch when OFF.

OE ---------|
_|_
_______
| |
Input _____| |______ Output

A CMOS buffer has a push-pull type output, with P-channel and
N-channel transistors at the output. When the output is high, the
upper P-channel is turned on, and lower N-channel is turned off. When
the output is low, the lower N-channel is turned on, and upper
P-channel is turned off. The outputs can be disabled using the OE
input.

VCC VCC
| |
|--- |---|
|-O| |-O|
| |---| | |---|
Input -----| |----| |-----Output
| |---| | |---|
|--| |--|
|---| |---|
| |
GND GND

Consider using a buffer if you are driving a heavy load or long
transmission line. You can use a transceiver function (SN74LV245A,
http://www.ti.com) for a bi-directional interface.

If you only need to channel a signal from the input to output, then a
bus switch would work fine. The propagation delays with bus switches
are very minimal which is an added advantage.

 

Wong,

Hi Prasad,
I am a bit of confusing. If a bus switch is just like a wire when
it's ON, then the signal should be uni-directional or bi-directional.
But you indicated that a bus switch would work fine if it is channel a
signal from the input to output(unidirectional).
I am considering a bus switch for the signals since I don't want to
control the direction flow with a control signal. If a bus switch
works in both directions, it will be the right choice.
Thanks Prasad for the pointer.

 

Wong,

Sorry to have confused you. You are correct - bus switches can be
used for bi-directional signals. I should've indicated "I/O1 to I/O2"
instead of "Input to Output"

--Prasad

 

Prasad,

Thanks a lot for the info.