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BS1 Serial Interface — Parallax Forums

BS1 Serial Interface

patdiamantpatdiamant Posts: 2
edited 2005-03-07 19:06 in BASIC Stamp
BS1 helped me very often to do nice things. I was using the parallel interface to programming with my Win98 PC. But last year his PS go to +10/+22v instead of +5/+12v, I don't know why (the "Magic Smoke" was gone!)smhair.gif.
So I buy a 2.4 Ghz PC·with XP from RS and I build your BS1 serial interface (with SMD parts from the fried PC) in the shell of a DB9 male connector and the cable of the parallel interface. Programming BS1 is working very well again now.turn.gif

But I don't understand something in the diagram of·this interface.
The voltages from RS232 ports are often swinging +/-10 volts, sometimes more, sometimes less for bad ones.
So the TX pin swing +/- 10 volts on the base of the "PCI" transistor via 10k resistor.
The·caracteristic of this transistor 2N3904 is:
Absolute Maximum Ratings··· VEBO Emitter-Base Voltage = 6.0 V
When the negative voltage go on the base, the junction is reverse biased and work in avalanche mode...
I don't know if this is·dangerous in this diagram.
I don't use scope to see what happens on this transistor, but I have already see erratic pulses on collector of so biased silicium transistors.
The solution is well known: a·reverse polarised diode between base and collector protect the junction.
Why not in this interface?
BS1 SerialAdapterSchematic.jpg
27K

Comments

  • patdiamantpatdiamant Posts: 2
    edited 2005-03-07 16:53
    Erratum

    ··· Hi,

    Because I receive no answer until now, I read again my post and I find·some errors. You must read at the end of the post:

    "...a·reverse polarised diode between base and·emitter protect the junction. See·new attachement for the suggested modification. My question is: Do you think this is necessary in this interface? ."
    BS1 SerialAdapterSchematicModified.jpg
    582 x 305 - 27K
  • Beau SchwabeBeau Schwabe Posts: 6,557
    edited 2005-03-07 19:06
    Your addition definitely will not hurt things, and perhaps you have found a small design flaw, But
    I don't think it constitutes as being a dangerous problem.

    The "Absolute Maximum Ratings VEBO Emitter-Base Voltage = 6.0 V" that you suggest is for
    forward bias conditions. With the current configuration this works out to about 1mA of driving
    current at the transistor base.

    simplified forward mode...
             10K   B    E
TX >-----/\/\---->|-----<GND



    Here there is only about .6V across the B-E junction

    simplified reverse mode...
             10K      100K
TX >-----/\/\--B--/\/\--E----<GND


    ....the two resistors form a voltage divider where -9V is across the B-E junction at about 90uA of current through the
    resistor divider.

    The data sheet characteristic that I MIGHT be concerned with here would be the Emitter-Base breakdown voltage
    giving a maximum rating of 6V at 10uA current through the Emitter, but since the transistor is off and there is no
    current through the Emitter, I don't see this as a problem either.


    As far as the Collector-Base breakdown voltage, I think we are safe here. The spec sheet says that this is 60V
    with 10uA current through the Collector, but again since the transistor is off, there is no current going through
    the Collector.

    If you follow the PC1 signal further into the BS1 module you will see that it is a CLK input to a uP which does
    have internal reverse diode clamping inside the chip.

    2N3094 DataSheet
    Other Side of the BS1 connector

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe - Mask Designer III

    National Semiconductor Corporation
    (Communication Interface Division)
    500 Pinnacle Court, Suite 525
    Mail Stop GA1
    Norcross,GA 30071
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