UTC cycle time adjustment

[Paul Rylatt]

I am attempting to use the RFC-1305 block in a program on a V130. Currently, the block cycles every 5 seconds. This seems somewhat excessive. I would like to make it adjustable so that the user has control over the cycle time and can eliminate unnecessary clock server calls. Another problem is that if NIST servers are used, pings faster than every 10 seconds are considered denial-of-service attacks and geographically remote PLCs without an intervening local PC clock would not be able to use UTC.

[linxchas]

Pooling NTP server every 5 sec is definitely too excessive and you are right about blacklisting clients for too frequent pings.

Most of NTP clients offering intervals in the range from 1 hour to 1 week.

Officially ntp.org says (http://support.ntp.org/bin/view/Support/SelectingOffsiteNTPServers):

"... once you're up and running you should not be contacting your upstream servers any more than once every 1024 seconds (or so), and this shouldn't be a problem for the time servers to support."

Regarding you second question:

Yes, NTP servers are geographically related.

If you want to avoid NTP addresses reference table for each PLC and reliable connection to your time source, try to use DNS resolver for NTP Pool servers:

0.pool.ntp.org

1.pool.ntp.org

2.pool.ntp.org

3.pool.ntp.org

The resolved IP address use for RFC1305 FB

"... The machines that are "in the pool" are part of the pool.ntp.org domain as well as of several sub-domains divided by geographical zone and are distributed to NTP clients via round robin DNS. Work is being done to make the geographic zone selection unnecessary" (wiki)