Prologix wrote:

I don't think the 3478A is sending anything at all.

If it was, it should be showing some of the annunciators on its LCD display,
like TLK, LSTN, RMT, SRQ, or something.  But it isn't.

I think the Prologix isn't capable of driving the 3478A, logic level wise,
and is waiting in vain for the 3478A to respond to a signal the 3478A cannot see.

There truly is a reason why 488 bus driver chips exist.  Something about the
talker/controller's 220 ohm/330 ohm pull up/pull down terminator spec, and the
38ma pull down spec.

Nope, doesn't work, locks it up on the first ++read command.

Sending T3 locks up the Prologix.

-Chuck Harris

And the answer is:

My Prologix USB <-> GPIB Version 5.0 adapter has insufficient
pullups to drive my HP3478A DVM.

Once I put another (older) turned on device on the bus, the Prologix
snapped to attention and works as it should.

Abdul, there is a very good reason why the 488 bus driver chips
exist.  The 488 standard requires that no listener load the bus
with more than 1 TTL input load, and requires that every active
Talker or Controller have a pull up/ pull down resistor on every
data and control pin that is equal to  this:

~    +5V
~      |
~    3.1K
~      |
~  Out-+---->
~      |
~    6.2K
~      |
~    GND

It would appear that my 3478A is in a non loading tristate mode
until it is addressed, and you are relying on the devices on the
bus to do their own pull up.  Clearly that isn't always going to
work.

-Chuck Harris

Chuck Harris wrote:

Hi Chuck,

Good work identifying the problem! Leaving out the bus driver chips was a
conscious decision based on available board space and was not made lightly.
Extensive testing had shown that not to be an issue, so far. In fact, yours
is the first reported case of a drive related problem. However, we consider
even one incident to be one too many, and will be revisiting the decision
during a future release.

Thank you for brining this to our attention.

Regards,
Abdul

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Chuck Harris
Sent: Wednesday, November 28, 2007 7:47 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] OT: Prologix GPIB and HP3478A...The Answer!

And the answer is:

My Prologix USB <-> GPIB Version 5.0 adapter has insufficient
pullups to drive my HP3478A DVM.

Once I put another (older) turned on device on the bus, the Prologix
snapped to attention and works as it should.

Abdul, there is a very good reason why the 488 bus driver chips
exist.  The 488 standard requires that no listener load the bus
with more than 1 TTL input load, and requires that every active
Talker or Controller have a pull up/ pull down resistor on every
data and control pin that is equal to  this:

~    +5V
~      |
~    3.1K
~      |
~  Out-+---->
~      |
~    6.2K
~      |
~    GND

It would appear that my 3478A is in a non loading tristate mode
until it is addressed, and you are relying on the devices on the
bus to do their own pull up.  Clearly that isn't always going to
work.

-Chuck Harris

Chuck Harris wrote:

to

unable

display,

cannot see.

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Yet it works fine with mine, even with or without a number of other
instruments on the bus, powered or not. I have not come across that problem
with either the Prologix of the IOTech Micro488/P which also uses a
microcontroller (Motorola 68HC705C4) to directly drive the bus (no GPIB
driver chip).

There has to be something else wrong, maybe the 3478 is partially defective
and requires too much drive, or is lacking the right pullup. The fact that
it works with the HP 85 indicates that it is not completely dead, but there
could be something wrong with it that makes it incompatible with the
Prologix.

I agree that it is always best to follow the standard, but in this case, I
doubt it is the root cause of the problem.

My HP 3478A has serial number 2619A36441, switch 6 (Power ON SRQ) is down
(0), switch 7 (not sure what it does) is up (1) and switch 8 (50/60 Hz) is
down (0)

Didier

Didier Juges wrote:

Ok, that pretty well rounds out the picture.  The 3478A uses the
75ALS160 and 161 bus drivers.  Both are designed to be either tristate
or pullup drivers depending on the signal on a single pin.  I will have
to put a scope on mine and see what it is being told to do.

Driver chips do fail... all too often, actually.

Switch 2 (what you call 7) is said by HP to be not used.  I have tried
mine in both positions, and see no change.

-Chuck Harris