Campbell CR800 Specifikace stáhnout pdf (Strana 292)

Section 14. PakBus Networking

14-8

would be marginal is acceptable. Allowing this link to be used would leave

the other link (through a router) as a redundant, backup link.

Using neighbor filters, you can determine which nodes will be considered

potential neighbors. This allows you to force packets to take a particular route,

the best route based on your knowledge of the installation. Other possible

links can be checked by temporarily changing the neighbor filters to beacons,

then switching back to neighbor filters.

With beaconing or neighbor filter discovery there is an important

consideration: links are established and verified using relatively short messages

(hello packets), but, when the links are used, longer messages are typically

sent. Consequently, a link could be reliable enough for discovery but

unreliable with larger packets. This condition could occur in an installation

that is subject to RF interference. The signal levels from the radio may look

ample (see RF401 series manual) but the link performance in fact be marginal.

Because of this, link integrity should be verified by using PakBusGraph Ping

Node.

The pings can start with 50 bytes, then 100, 200, and 500. Doing 10 pings of

each packet size will characterize the link. Other network traffic (scheduled

data collections, clock checks, etc.) should be temporarily suspended while

doing this test. The pings should start from the PC base using PakBusGraph

going to neighbors (1 hop away) then proceeding to nodes that are more than 1

hop away. The performance gage: 10 of 10 is good; 9 of 10 is still good; less

than 7-8 of 10 (500 byte packets) would enter the less reliable area.

When beaconing in a network, keep beacon intervals as long as possible with

higher traffic (large numbers of nodes and/or frequent data collection). Long

beacon intervals minimize collisions with other packets and resulting retries.

The minimum recommended beacon interval is 60 seconds. If you have higher

traffic, you should consider setting beacon intervals of several minutes. If data

throughput needs are great, you can maximize data bandwidth by creating

some branch routers (see 14.3.7), and/or by eliminating beacons altogether and

setting up neighbor filters.

Configuring a neighbor filter in a router consists of (1) inputting the PBAs of

Neighbors Allowed and (2) inputting Verify Interval xxx for the applicable

port. Take, for example, the first CR800 router in the above PBA assignment

network. If you needed to set up a neighbor filter, in the Neighbors Allowed

SDC7 field you could input “4094, 11, 12, 13, 20”. For the CR800’s Verify

Interval SDC7 field you might input ”120” for a 2 minute communication

verification interval. The “4094” isn’t necessary because PakBus addresses ≥

4000 penetrate any neighbor filter anyway.

If a neighbor filter’s verification interval expires without normal

communications (such as data collection), the router tries to reestablish

neighbor status by initiating a hello exchange. A beaconing device’s

verification interval is its beacon interval × 2.5. So, a beaconing CR800 needs

no Verify Interval xxx setting (it can be left 0). If verification intervals

between neighbors are different, the lesser verification interval is used for the

link as necessary to try and maintain the neighbor status (see PakBus Concepts,

Verification Intervals).

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