Question about 74LS245 bus transceiver

So, I'm sitting and reading the 74LS-databook just for the sake of getting
a wider view of what's available out there, which in turn hopefully learns
me more about electronics.

I got stuck on 74LS245, an octal bus transceiver.

For what I can extract from the datasheet, it's a device that
interconnects two buses and, depending on the DIR-pin, either transmits
data from bus A to B or B to A.

Forgive my ignorance, but my question is; when do you need a device like
this? Why not simply connect bus A to B?

 

74LS245, like most bus-oriented chips, has tri-state drivers. In the
third, high-impedance state, the driver is effectively disconnected,
allowing other drivers to use the bus. If you just connected the busses,
you could have only one data source.

 

Imagine decoder chips that drive 7 segment displays.
This is data out used with a chip enable.

Imagine latches with information ready.
This is data in with a chip enable.

micro hooks up to one side of the 245, and all other I/O devices hook up
to the other.

This method allows you to drive any number of I/O devices simply by
selecting direction, and asserting Chip enable for each additional device.

Data sheets will give you the timing needed to read/write to I/O.

Don...



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Circa Tue, 08 May 2007 07:34:01 GMT recorded as
<JJV%h.40370$> looks like Rikard Bosnjakovic

When there exist busses C, D and so forth. This device is meant to be
controlled in such a way to permit communication between internal busses
and a common data and/or address bus. So call the data bus bus D. Bus A
needs to read data from bus D. Enable the 245 to permit bus A to receive
data from bus D, while disabling the transceivers on the other busses.
Likewise for B, C etc. Reverse the direction when bus A has data to place
on bus D. Rinse, repeat.

Another example would be data routing. Enable bus A to transmit to bus D,
and bus B to receive from bus D. Data now is connected to pass from A to
B. Shut off bus B and turn on bus C to receive and.... Get the picture?

 

Think of it as an amplifier. Say you have a CPU. You can connect the
databus directly to some memory, but once you start adding a lot of circuitry,
it would load down the CPU's data bus too much (the CPU datalines couldn't
supply enough current). So you add a buffer so there won't be much load on
the CPU, but there will be enough signal on the databus for all the
peripheral circuitry.

It's a transceiver because the databus is going both ways. You could use
two sets of one-way buffers, to buffer signals out of the CPU and to buffer
signals going into the CPU, but that would be bulky. There is enough use
for this that it was worth creating a single device for it.

And given that many of the answers here have addressed the tri-state
nature of the device, you can also turn off the transceiver, so the CPU
databus is isolated from the rest of the circuitry. Not so useful
in itself, but if you had two CPUs using the same rest of the circuitry,
disabling the databus transceiver connected to one (as well as the buffers
on the other CPU's lines) would allow the other CPU to take over. WIthout
the bus transceiver, they would fight over the bus.

Michael

 

Charlie Siegrist wrote:

[...]

I got it perfectly. Thanks.

 

Two important reasons:
- The tranceivers do not only connect/disconnent busses, they also buffer
(refresh) the signals. Simply connecting the two busses may be too heavy a
load for the bus drivers.
- You may have three or more busses. Example: You have bus A, B and C and
you want to be able to connect A to C or B to C but never A to B as they can
be both outputs at the same time.

petrus bitbyter

 

maybe sometimes you want A and B to do different things.
and only communicate occasionally,

maybe you have 10 As and one B which you want to share between the As.


Bye.
Jasen

 

On a similar note, I have used the 74LV4245, which is similar. The
4245 is a bi directional, octal bus transceiver, like the 245, but it
allows for different voltage potentials on the A and B side. I use
the 4245 to connect 5.0V devices to a 3.3V system bus.

 

Circa Tue, 08 May 2007 14:53:38 GMT recorded as
<S900i.40396$> looks like Rikard Bosnjakovic

Sweet!