Allen Bradley Data Highway Plus Traffic

I have an existing Allen Bradley Data Highway Plus network I am expanding. It there a way to tell current traffic volume (i.e. 75% fully loaded)? I am limited in baud rate by one of my devices but could split the Highway in two if necessary.

Thanks, BobbyH

DH+ uses a peer to peer "token passing" protocol to determine which node on the network has it's turn to be the messaging master. From the users point of view each node gets an equal turn at initiating messages, so they all appear as peers, although in reality they are all simply taking brief turns at being masters.

This means there is always traffic on a DH+ network, even when there is no useful data being sent, the DH+ protocol is busy passing token packets around each node, so in effect the network is always 100% loaded.

Useful messaging data is transmitted once each node has secured "mastership".

As you add more nodes the "overhead" involved in this "token passing" (this isn't the strictly correct term for it, but it's always how I visualised it) becomes larger and larger.

So there are two effects on DH+ throughput...how many words/second, and how many nodes.

A two node network will cheerfully handle 1000 or so word/sec, but a 64 node network might barely handle 50 or so.

In practise AB recomend an upper limit of about 16 nodes on a single DH+ segment. Any more than this and the network overhead tends to dominate the network performance.

Below 16 nodes and your decision really depends on what kind of thruput times you need. If you have several SCADA nodes hammering away the the PLC's then perhaps only 2-3 nodes is ideal, on the other hand I've seen 15 or so nodes on a VERY busy network work with only moderate 1-2 second delays.

Depending on what your needs are you may be able to utalize the global data aspect of the DH+ network. The global data allows you to produce one word worth of data to everyone on the network as part of the overhead during the token passing. This is a very easy way to send/receive data from one plc to all of the others on the network without the use of MSG instructions. The only draw back is the one work limitation.
I currently have 14 nodes at 57K with a SCADA computer and each PLC uses the global data word plus two or three MSG instructions and I have seen no delays and SCADA updates fine. Most tags in the SCADA are polled every one second.

Good Luck,

Global data

yes, global data is a highly under-utilized resource that most
present-day PLC 5 types have simply not learned about.

You can use it as a 'benchmark' measurement tool, if you think
a little bit.

You'd have to set up two PLC's to read each other's global data
word, and toggle bits, and simply time the period between toggle.

In this way, in a crude way, you could at least get an idea of
the relative loading of the DH+, by watching the exchange times
between two stations.

Or, you can 'zero out' the contents of another station's word, and
track how much time it takes to have the highway 'restore' it to its
correct value. Same concept, different measurement.