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

[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:
> 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.
>