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Question about 74LS245 bus transceiver

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, May 8, 2007.

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  1. 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?
  2. 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.
  3. Don McKenzie

    Don McKenzie Guest

    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 McKenzie
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  4. 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?
  5. 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.

  6. Charlie Siegrist wrote:

    I got it perfectly. Thanks.
  7. 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
  8. jasen

    jasen Guest

    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.

  9. Noway2

    Noway2 Guest

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