>The gate time on a 5335 can be set via GPIB. It can be set to some very long gates (well past 100 seconds).

 
Hi Bob,
 
You need to use timed GO and GC  commands (Gate Open and Gate Close), for > 1sec Gate Time.
The GA command (Gate Adjust) is restricted from 1ms to 1 sec Gate time, so will not do the job.
 
See manual page 3-29.
 
One error correction: I replaced this 10µF Gate Time capacitor with a Tantalum type, 33µF, obviously, as these original 10µF already give about 4sec, so the 33µF are good for 12.5 sec, 10 digits.
 
 
The designers very well designed two near inifinite counter chains inside the 5335A.. I got my wisdom from the hp journal 9-1980.
They used a digital ASIC, called the MRC, multi-register counter, and it allows counting of the input signal and the time base, each 20 decades long. (see page 26).
 
Therefore, the Gate Time could be 300 000 years long, before an overflow from the 10MHz time base would occur, and the result would theoretically be precise to 20 digits resolution plus 2 digits for the time interpolator.
So you will NEVER loose any MSB information.
 
The GPIB output also does not stretch its resolution , because in the manual, they have a fixed digits output format.
On second read (pages 3-32 .. 3-34), it's not limited to 9 digits - sorry for the confusion - they reserve 14 digits for the mantissa, including sign and comma, so up to 12 digits can be transferred also, like on the display.. that means, also 11 significant digits over GPIB.
 
I assume, they internally use about 13 digits for the arithmetic routines, maybe taken over from the HP pocket calculators. 
 
Frank

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Hi

They may have intended to have infinite resolution. The net result in the real counter is that you get
MSB rollover. The LSB’s are correct pretty much forever and ever.

The same math tricks mentioned before can be used to get high resolution on the display once you
get the gate running.

Bob

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1980-09.pdf

Just to be clear, the MRC itself has only 8 decades. You get additional decades with off-chip processing, which is to say, counting overflows. It's the same way we keep 64-bit time in a microcontroller: 16 bits of timer and then 48 more bits of overflow count.

"Another feature of the MRC lets its internal eight-decade
count chains be expanded to any number of decades with
the help of a processor. This feature is used to create a pair of
count chains, each 20 decades long. With a 100-MHz signal,
this chain would take more than 30,000 years to overflow.
Consequently, the 5335A has no overflow annunciator, so
gate times can be as short as 0 ns or as long as years."

There's more good info on the MRC starting on page 12 of an earlier HP journal:

"A High-Performance Bipolar LSI Counter Chip Using EFL and I2L circuits"
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1979-01.pdf

The heritage of the MRC shows up in my picPET (Precision Event Timer). Notice how hp's MRC also has an E-register (Event) and a T-register (Time). Once you have matched {event count, time stamp} pairs anything is possible. By "anything" I mean, totalizing, absolute time, time interval, frequency, period, mean frequency, period average, best fit frequency, sigma, sigma(tau), ADEV, etc.

BTW, if any hp old-timers reading this posting happen to have detailed information on the 40-year old proprietary MRC chip, please contact me off-list before it's too late. By too late I mean:

http://www.pressdemocrat.com/business/7559762-181/hewlett-packard-archives-at-keysight-destroyed

/tvb