Types of digital outputs & their ID

Discussion in 'General Electronics Chat' started by SPQR, Feb 24, 2013.

  1. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Me again,

    I'm learning a bit about buffers/drivers, and after reading a little on the net it seems that the types of outputs can be divided as follows:

    Open collector
    ____current sinking - NPN - needs pullup resistor
    ____current sourcing - PNP - needs pulldown resistor


    Push-pull
    ____Totem pole - similar transistors
    ____Standard push-pull - different transistors

    Tri-state

    When I read datasheets, they will often say "open collector output", but I've found some where there is no specific type stated.
    Example: SN75155 - there is no specific statement of the type, and they give a schematic which shows an output between diodes. So it doesn't really fit into the above schema. (What's up with that?)

    Questions:
    1 - If a datasheet doesn't state "open-collector" is there an assumption I can make about the type of output (in the absence of a schematic).

    2 - In a tri-state buffer (e.g., DM74LS125A) what type of output can I assume it has when it is either HI or LOW? (not Z)

    As usual, I thank you in advance.
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    if i'm not forgetting something, all 3-state are totem-pole with "disable".
    your link to
    http://www.ti.com/lit/ds/symlink/sn75155.pdf
    shows everything one could want, even internal circuit schematic.
    there is a push/pull or totem pole output. look at the logic diagram, DY is inverted DA, there is no tristate here.

    1. no, read the datasheet
    2. look at the diagram and state table. when not tristated, output is same as input (this chip is four non-inverting buffers).
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    The diodes you see going to the supplies are protection diodes and they don't come into play unless the output is taken more than a diode drop outside the range of the supply rails.

    This is a pretty classic totem pole output output.

    The '125 is basically a normal logic drive output when it is enabled. Look at the truth table and you will see that the outputs are listed as H and L when enabled. That means that the output is actively being driven to that level.
     
  4. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Thank you!
    Yes, I included the 75115 as an example of a non-three state without open collector - the output type wasn't clear since I couldn't see an "open collector" sticking out of the end:)
    Thanks for responding.

    Nice.
    So the key to "totem pole" is just two similar transistors "near" :) the output - irrespective of diodes.
    Your comment about the "protection diodes" is interesting - I'll look that concept up.
    Also your phrase "actively driven" will help me in my searches. (so I'm guessing "open collector" is "passively driven")
    Some datasheets don't have a circuit diagram, but I'll bet if I search around I might be able to find one somewhere.

    Excellent answers as always, and thank you both for your help!
     
  5. panic mode

    Senior Member

    Oct 10, 2011
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    totem pole is pair of transistors "on top of each other"...
    they do not work at the same time, when top transistor is on, you get high level out. when the low side transistor is on, you get low level out.

    some MCUs allow selection of "normal output" (totem pole), or open collector/drain. in this case (open drain) the top sode transistor is simply disabled.
     
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  6. takao21203

    Distinguished Member

    Apr 28, 2012
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    MCUs don't use BJT even if they are integrated circuits based on BJT.

    Early CPUs used pMOS then this evolved into nMOS, CMOS, HMOS, and more modern variants.

    You can read about that on wikipedia, even if the pMOS article is a bit short.

    Some MCUs actually do have a current limit, I think done with MOS transistors as resistors. CPLDs even have programmable driving strength, selectable open collector etc.

    Most chips have active drive, and tristate is also common (so called output enable). Open collector is used for special purposes.

    The protection diodes are also often called clamping diodes. They are not automatically present in all chips.
     
  7. SPQR

    Thread Starter Member

    Nov 4, 2011
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    So, again, it's just a question of delving deeper into the datasheet, and if it's not there, look elsewhere.
    Thanks!
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    If an output is open collector it would clearly say so.
    If an output is tristate it would say so and there will be an output enable pin.
    Otherwise the output is totem-pole.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Almost universally when an output is stated as an open colector it is the sinking type. This beacuse this type allows to communicate between devices with different voltage levels (if you take some precautions regarding overdriving the ouptut with higher voltage), whereas the sourcing type doesn´t really have much application, so it would most definitely be stated in the datasheet.
     
  10. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Ok, nice...a "rule" to remember.:)
    The "tristate" I understand pretty well - the concept of an "enable" line allowing a circuit to work.
    But now I can say that a "tristate" is a "totem-pole" with an enable line.

    I've got a spreadsheet with 20 drivers/buffers, and I'll run through those today to see if there are any unidenfied types of outputs.
    Thanks!
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    The number of 7400 series chips with open collector and open emitter is almost zero.
    Hence you can ignore these.
     
  12. takao21203

    Distinguished Member

    Apr 28, 2012
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    There are some older ICs which were used for VFDs/other high voltage displays, with open collector.

    They are no longer used commercially. They are even no longer sold. Or are there a few still around?

    CPLDs however do have configureable open collector.

    Some PICs also do have at least one I/O with open collector.

    Almost always for current sinking. I am unaware of p-channel current source if you neglect LED drivers, LCD drivers and some special consumer products ICs.

    Not digital ICs, for instance motor driver ICs.

    a "tristate" is a "totem-pole" with an enable line

    No. Tristate has no correlation to the type of the output. It simply means the output can be put so-called "High-Z".
     
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    And it can be put to two other states. With your definition you could call an open collector tri-state, which clearly is not true.

    I am not too sure about the definition, but I see totem pole as a configuration with an upper switch and a lower switch, so a tri-state is then a totem pole with a disable capability.
    Yes there could be some special case where you could have an upper switch, lower switch and a switch in series with output, or have three states like -5, 0, +5 and that probably wouldn´t be totem pole as well, but those are so weird combinations that I doubt they were ever used.
     
  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    Normally /OE is used.

    There are a few older chips which use Output inhibit (these are display drivers).

    Never saw /OD (for output disable).

    All this stuff is pretty standard you'd get used to it if you'd be up to deal with such chips in circuits.
     
  15. WBahn

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    Mar 31, 2012
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    The terms "totem pole" and "push-pull" are used synonomously by some people and used to mean different things by other people. When you have two transistors of the same flavor, some people call that a totem pole configuration and distinquish it from transistors of oppositve flavor which they then call a push-pull configuration. Other people don't make the distinction and use either term to refer to either configuration. I've never tried to look up which is "correct" and it doesn't really matter because both ways are in sufficiently common usage that you've just got to deal with it.

    The term "active outputs" generally means that it drives both HI and LO while three-state (tri-state is actually a copyrighted term) outputs are active in one direction (usually active-LO) and passive in the other (usually requiring a passive pull-up).
     
  16. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Excellent comments by all.
    Tonight, after work, I'll post all of the drivers/buffers I've been looking at, along with what information I could gleen from the datasheets.
    I think you'll see where my problem lies.
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Connecting to a bi-directional data bus, open collector outputs are often used (though tri-state is an option as well). This allows any device on the bus to "pull it low" to get the attention of the others, if that makes sense.

    With a differential bus, where two wires are used for sending and two wires for receiving.
     
  18. WBahn

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    Mar 31, 2012
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    Another thing to be aware of is that many chips have an "output enable" input that can lead you to believe that they are three-state outputs when, in fact, the output is driven to one particular state (usually LO) when the output is disabled. This is nothing more than a normal active-output that is gated with the output enable signal via an AND (or an OR if the diabled output state is HI). These are meant to be connected to other gates via an OR/NOR gate (or and AND/NAND if the disabled output is HI) and not connected to each other directly. You often see these in FPGA component libraries and the like.
     
  19. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Ok, here we go.
    It all started with THIS thread, where I asked if there were certain "general solutions" to connecting stages together - learning that every component in every circuit needs to be considered as a part of a whole.

    So I've been collecting "drivers/buffers" and want to learn more about them. One way to learn is make a list and compare one to another. A spreadsheet was built, and now I'm trying to fill it in.
    Across the top I have:
    Number Description Units per package Output Output type Inv/Non-Inv Op Voltage#Loads/pinOutput IOutput VSink IMax Coll VLoad Type
    I'm mostly focusing on Output type = open coll/totem/etc

    The vertical axis is:
    74HC125
    SN7417
    SN74HCT540N
    SN74HC05
    SN75155P
    74LS541NCD
    74HCT541E
    SN7417N
    74HCT244
    L293DU
    LN2803A
    U
    LN2003

    7406
    SN74LS541
    SN74LS244N

    74HC125 - HERE -three state - no output type state, no schematic - open collector or totem?
    ULN2003 HERE - says "common cathode" but looks like open collector on circuit
    74LS541 - HERE - by circuit - totem
    7400 NAND - HERE - totem by circuit

    So based on all of your previous comments I've been able to fill in a few boxes.
    Any other hints about identifying output types?
    Thanks
     
  20. WBahn

    Moderator

    Mar 31, 2012
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    I think you can safely assume that any three-state output is NOT also open collector. First, there wouldn't be three states, only two! If there were some chip that had such an output for some particular application, you can bet it will spell out in clear terms what is going on.

    The common-cathode diodes they are talking about are clamping, or anti-kickback, diodes for use with inductive loads. It looks, to me, like the protection/parasitic diodes they show connected between the input (and the output) and the E pin are backwards. You want them to turn on if the input/output go more than 0.7V below the E voltage. As drawn, they would turn on once the input/output go more than 0.7V above the E voltage, which basically makes the circuit worthless.
     
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