[time-nuts] clock-block any need ?

Jim Lux jimlux at earthlink.net
Fri Dec 28 15:21:26 UTC 2012

On 12/27/12 11:48 AM, Attila Kinali wrote:
> On Thu, 27 Dec 2012 11:30:37 -0800
> Jim Lux <jimlux at earthlink.net> wrote:
>> So, what about the USB-Ethernet dongles?  (I use them a lot at work to
>> add a second interface for a laptop in test equipment setups, talking to
>> a Prologix, for instance)
> A lot worse! One thing is that USB has a polled protocol (ie the host
> has to ask whether the device has any data) and the slot when an USB
> device can signal that a packet came in repeates it self at most a couple
> of times per 1ms frame for FS (don't know from the top of my head for HS).
> The bigger issue though is that drivers for ethernet dongles are usally
> of quite bad quality (ie it takes them a long time to do anything), and
> they try to queue as much as possible to increase troughput (which adds
> unpredictable delay).

OK, so throw out the USB version..

Just thinking out loud here: are these $20 wonders potentially a way to 
get "better" time transfer via Ethernet?

I can see that a PC interface version might not be very good (because of 
the need to build a low jitter driver for the PC for SPI or RS232).

But what about connected to some purpose built widget.. That is, rather 
than invest in a IEEE 1588 interface, could one cobble together 
something that does somewhere in between the vanilla mobo Ethernet & 
network driver that came with the OS on the "high jitter" end and some 
IEEE 1588 rev2 on the "low jitter" end.

After all, there's no real "driver" when you're hooking it up to an 

There might be some significant uncertainty in the microcontroller 
inside the interface chip: it does quite a lot.  But maybe not.. I 
haven't looked at it, and I was just curious.

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