[time-nuts] Timelab, two SR620s and losing samples

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jan 17 07:38:06 EST 2016


Poul-Henning,

On 01/17/2016 12:52 PM, Poul-Henning Kamp wrote:
> --------
> In message <569B61CF.3030708 at rubidium.dyndns.org>, Magnus Danielson writes:
>
>>> This is a common misunderstanding:  Talk-only does *not* protecting you
>>> against timing issues on GPIB.
>>>
>>> On RS-232, yes, but not on GPIB.
>>
>> Agree, to some degree. It's not a guarantee.
>>
>> I think you should develop that line of thought, to detail why it helps
>> on GPIB and why not on serial.
>
> It's really very simple:  RS-232 sends blind, you don't even need to
> know if there is a receiver or what it does.  If the receiver cannot
> keep op, data is simply lost.
>
> GPIB handshakes every byte, so the actions of the receiving end affects
> the transmitting end - in particular if the receiver cannot keep up.
>

OK, you where thinking about the flow-control.

You can have RS-232 wired up to do flow-control (hardware-flow-control), 
where as flow-control is a standard property of GPIB. On the other hand, 
flow-control in itself only assures the data-transfer but not that 
triggers is not missed in the overall system.

I just didn't want to mind-read you on this one.

So, I don't see how flow-control would severely break in GPIB doing 
addressed mode compared to talk-only mode, it's an orthogonal feature of 
both.

It is however to some degree easier to control the real-time flow from 
only one instrument in talk-only mode compared to addressed mode. 
However, it should be possible to achieve it also in addressed mode, but 
you then need to think through the signaling interaction. Also, as both 
counters is triggered, the trigger interaction needs to be analyzed and 
the dataflow understood in the system context.

The SR620 does not recommend the AUTM1 mode for serial interface, as the 
synchronization between program and data-readout does not become as 
easy. Similarly, the SR620 does not support the binary mode for serial 
interface, but for GPIB, giving almost a ten-fold increase in 
sample-rate (150 vs 1400 samples per second).

Anyway, flow-control to ensure integrity of byte-flow should always be 
used. That will help if we can trigger both counters at the same time 
and guarantee that they are idle before next arming pulse.

Cheers,
Magnus



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