Subject: Are 7438s in DEC interfaces special?

From: Ethan Dicks <[email-masked]>
Date: Fri Jun 27 20:49:26 2003

I need to lay hands on at least one 7438 to stick into my new (to 
me) Emulex QD21 to implement the "22-Bit Addressing Kit"... there's
a single socket by the fingers, U49, that accepts the 7438, and a
switch to throw.  I can pick up a 7438 for all of $0.28 at B.G. Micro,
but the question is are they hand-picked in any way?  If so, what would
I have to construct to replicate the selection process.

Thanks,

-ethan

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From: "Eric Smith" <[email-masked]>
Date: Fri Jun 27 20:50:57 2003

> I have to construct to replicate the selection process.

I'm not sure about Emulex, but DEC qualified all their Omnibus/Unibus/Qbus
drivers and receivers for leakage current lower than the manufacturer's
specifications, and for better Vol specifications at high current.  This
is necessary if you have a long bus, such as two Omnibus boxes with two
backplanes each, etc.  If you don't have a lot of loading on the bus,
it's not too critical.

To measure leakage current, make sure the output of the 7438 is NOT
being driven (for instance, by setting both inputs low), then connect a
microammeter between the 7438 output and +5V.  Don't do this with the
7438 actively driving the output low, as you could damage the meter.

For Vol measurement, you just need to set up a 70 mA regulsted current
source from +5V to the 7438 output, and set both inputs of the 7438 high
so that the output is driven low.  Then measure the output voltage.  A
fancy circuit to do this, allowing you to dynamically control the
current, may be seen in a Maxim appnote:
    http://www.maxim-ic.com/appnotes.cfm/appnote_number/177
However, for just a fixed 70 mA current, you can use just a linear
regulator with a resistor, as shown on page 4 of National Semiconductor
application note AN-103:
    http://www.national.com/apnotes/PositiveVoltage-Fixed__v3.html

Note that the leakage current is a concern on both bus drivers and
bus receivers (or transceivers).

Also note that the leakage current and Vol vary with temperature and
supply voltage, so if you really want to do this right you need to
test at the limits.  I suspect that the worst-case corner is high
temp and max voltage, but I don't know that for a fact.

I haven't actually tried doing any of this, so take it with a grain
of salt.

---

From: [email-masked] (Tony Duell)
Date: Fri Jun 27 20:55:10 2003

> but the question is are they hand-picked in any way?  If so, what would

I think DEC selected them (for leakage, drive, etc), but I don't think 
Emulex did (or at least my Emulex SCSI card came with a 7438 to insert if 
you wanted 22 bit addressing and it didn't have any special markings). 

Unless you have a well-loaded bus, I think any 7438 will work.

-tony