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MC678P chips needed.

Discussion in 'Electronic Design' started by Jamie, Feb 22, 2013.

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  1. Jamie

    Jamie Guest

    If any one has a tube of MC678P's they would like to part with, the
    company would be most willing to buy them..

    We need them to keep an original, first one every made, irradiation
    unit operating to its closes original state.

    Recently I was informed that they have been having some issues with
    arcing in the vessel or some where, and they have almost depleted the
    stock of replacement hex inverter chips. This is on a board that is part
    of the interlock and voltage monitor and drives reed relays, which in
    turn, drives larger relays.

    Looking around I find the chinese market are more than willing to
    sell us some. But our finance department really does not want to go this
    route if possible, we would rather buy from a US supplier or just some
    left over stock some one wants to unload.

    In any case, we rather not use NTE.. We want MC678P marked parts.

    I've already got a daughter board in the works, that will use some
    bipolars and will plug directly into the 16 pin dip socket, if it
    comes down to that.

    Thanks.

    Jamie
     
  2. josephkk

    josephkk Guest

    Try reposting to s.e.repair and s.e.components.

    ?-)
     
  3. There are folks out there who can mark whatever numbers might match
    your request on "probably close enough" parts, just to give you that
    feeling of confidence.


    Best regards,
    Spehro Pefhany
     
  4. Jamie

    Jamie Guest

    We found some in the US, 16.50 each.. we have 25 coming...

    Thanks..
    P.S.
    Most of the repair guys seem to think EGC, NTE or what ever
    is close enough. We don't deal that way, we want original numbers
    to insure we have the correct part.

    Plus we needed a tube of them, not just a couple which is why I
    thought maybe some old timers here had some in their bins, I know I
    have a collection of hard to fine chips. ;)

    Jamie
     
  5. Jamie

    Jamie Guest

    Yes that is true however, I spent hours looking at old logic of
    various families and I could not find one that matches this one, this is
    an odd ball part.

    So basically it is a 6 unit hex inverter with 4 units using a single
    enable and the other two using a single enable line, simply by combining
    the gate inputs of (NAND gates), into one.

    On top of that, it operates on 15 volts and drives reed relays
    directly from the outputs and has open collector outputs.

    Putting all that in perspective, I don't think there are too many
    different devices in a 16 DIP P package that would replace this.

    Unless there are some gate arrays that can be programmed to do so :)

    We'll see tomorrow when the tube arrives!

    Jamie
     
  6. Guest

    Aren't P-Dips obsolete?

    15V? PLD? You gotta be kidding.
    Though some is still used. I rather like the SC-70 1G parts. I'm
    even designing in a 74HCT14 into a board, now. I need 5V drive from a
    3.3V CPLD (dumb regulator needs 5V on its clock input).
     
  7. Jamie

    Jamie Guest

    I find hanging a open collector output being driven from what ever
    logic family you want to use works nicely. The devices on the outside
    can use their own pull up if needed, at various voltage. Of course, you
    need to NOT (!) the logic for that.

    Jamie
     
  8. Jamie

    Jamie Guest

    That's ok Jim, you can keep your chips. I don't want to become
    one of your elusive pals.

    I rather spend the money and not kiss your feet.

    Jamie
     
  9. ISTR some chips have caveats on how different the output voltage can
    be relative to Vdd. IOW, it's okay if you derive the 3.3 from the 5V
    but if the 5V pullup is applied with Vdd too low the chip might not be
    happy.


    Best regards,
    Spehro Pefhany
     
  10. Jamie

    Jamie Guest

    Not sure about what you're asking her but, If you were to employ a OC
    output into a design, one needs to invert the logic at the MPU.

    Of course the idea is to not have to worry about external voltages
    making their way to the low voltage bus using OC. Also it serves to
    allow you to pull down other devices that can be energized via low side.

    We use encoders with open collectors. Some times they are PNP (source)
    or (NPN) sink. The sink (OC NPN) can connect to any digital device of
    various voltages, normally up to the 24 volt level when the encoder can
    only handle 3.3 or 5 internally. If the receiving device requires the
    input to follow it's logic level voltage state, then a mere pull up will
    do. if the pull up does not have enough speed, you can then put a PNP on
    the input of that receiving device.

    AS for PNP open collector, they are not so popular, mostly used where
    input devices require source signals. In that case, you need to make
    sure the encoder or prox etc is operating at the same level of supply.

    Devices that can trigger on sinking the input are more desirable.

    Some times we use darlington components because of the high beta and
    they match 3.3 logic very closely for base drive, they also have a diode
    in the body that helps for reverse voltages. Then we put a low voltage
    zener at the base to common. This removes the unwanted transient from
    the miller effect and you don't need high levels of voltage entering the
    bus.

    Jamie
     
  11. Guest

    As long as you don't care about symmetry or rise time, it works.
     
  12. Guest

    Iff it's an inverting gate. The same applies to a totem pole driver.
    But it gives a crappy rise time, if that's important. Unfortunately,
    in my case the rising edge is the important one.
    I use PNPs all the time. The sex if the driver is determined by the
    more important quiescent state. If the low state current is
    important, for example, the signal is low under battery power, a PNP
    makes a lot of sense. PFETs work, too, but they're about 10X the
    cost.

    As you note, sometimes you need to source current, too.
    I suppose. NPNs save a half cent.
    Darlingtons cut way into the noise margin in the low state, though.
    Ick! If I needed to save that much current, I'd use a FET.
     
  13. Guest

    Is the particular application even using those strobe inputs? If not, thereare a bunch of inverters in that family to choose from. Also, regardless, there is the same layout in another family member that has resistor outputs, that's a 15K pull-up which should also work in the same circuit, an extramilliamp isn't going to break the bank. And then everything in that familyexcept of a few parts with active pull-up, can be wire-OR-ed if that's what they're doing.
     
  14. Jamie

    Jamie Guest

    Yes, the enable inputs are being used.

    There is a master ready signal that enables the ENABLE line on the
    pair, one of these pairs then drives the ENABLE line for the other 4.

    Mean while, the inputs to these inverters are receiving ready active
    signals from different points.

    There is a sequence of events that must take place in order to
    insure proper operation before starting the units.

    Jamie
     
  15. Guest

    I'm really having trouble understanding how a negative logic AND output can function as a positive logic input enable for the other ANDs.
     
  16. Jamie

    Jamie Guest

    Because the Master readly line is a sinking type?

    ion gauge __ ---- o\
    +----------------------+| \ nnnn o
    15V ___ | )o+---------+| +-----
    -|___|+-++----+|__/ +
    Low Limit | __ ---- o\ |
    ------------------+| \ nnnn o |
    + | | )o+---------+| | +| 15V
    +-----+|__/ +-----+-----------+
    | |
    HV on | __ ---- o\ |
    +-----------------+| \ nnnn o |
    | | )o-+--------+| | |+
    +-----+|__/ +-----+
    |+ |
    Safety Ready | __ ---- o\ |
    +-----------------+| \ nnnn o |
    | | )o+----------| +-----+
    +---+-+|__/
    |
    |
    |
    |
    Enable |
    o-o----------+ +-------------+
    | __ |
    | +--+| \ |
    ___ | | | )+---+
    15V -|___|--+--------+--+|__/


    MC678P
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)



    Jamie
     
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