[time-nuts] OT: Prologix GPIB and HP3478A...The Answer!

Didier Juges didier at cox.net
Wed Nov 28 22:55:36 UTC 2007


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 

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Chuck Harris
> Sent: Wednesday, November 28, 2007 9:47 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:
> > Prologix wrote:
> >> Hello Chuck,
> >>
> >> It appears that 3478A sends data continuously as soon as it is 
> >> addressed to talk. Since the Prologix adapter is busy 
> processing GPIB 
> >> data it is unable to respond to USB commands.
> > 
> > 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.
> > 
> 
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