[time-nuts] Re: Ryzen mobos with serial port for Garmin GPS?

[Rich Wales]

Angus wrote:
> /What timing issues did you have? I've used a similar setup on two old 
> windows machines (one XP and one Win10) and I didn't see much 
> difference between the built in serial port and the ones on the PCI 
> serial cards. They were not PCIe./

The serial card I used (with unpredictable PPS signal timing) was a PCIe 
card.  But I think the more significant difference was that this PCIe 
serial card was really a USB card with a serial-to-USB front end, and 
this protocol conversion was the reason for the extra jitter.

Old PCI serial cards came from a time before USB was common (or even 
existed), and they sent the serial data directly to the PCI bus without 
going through any intermediate steps.  If we could find PCI or PCIe 
serial cards which worked in this same way, we would presumably have the 
same high performance as in the old days. However, there is very little 
incentive nowadays for hardware manufacturers to do this; the few serial 
cards that are still made today (after all, who really needs, wants, or 
even knows about RS232 anymore?) generally cut corners by slapping a 
cheap serial-to-USB front end onto a cheap USB card, and except for 
those of us who really need accurate timing, none of their customers are 
likely to care or even notice.

*Rich Wales*
richw at richw.org