Data collection tips

Construct your processor data structures so all data table addresses are read from or written to via DDE are contiguous. All DDE requests are optimized around the size of a highway/network packet. If data is organized contiguously, block reads or block writes can be used to gather the data. This is more efficient than reading from or writing to a number of addresses scattered about a data table. For example, if you need to read 10 integers from a PLC-5, it is best to read N7:0,L10 rather than 10 separate integer words like: N7:1, N10:100, N21:0, N22:3, N23:10, N11:0, N27:23, N33:45, N37:302, N44:7. The data can be read either way, but it is faster to read it in a block. This also applies to DDE writes.

Use more than one topic when using hot links to read a lot of data that you do not need to have read all at once. Each topic is configured for a different poll rate. For example, you may need to read 500 integer words, but you only need to get 100 of those words every 2 seconds. You could set up one topic to poll at 2 seconds and use it to establish the link for the 100 words, and set another topic to poll at 10 seconds and use it to get the additional 400 words.

The DDE/OPC item modifier

noopt

allows the client rather than RSLinx Classic to optimize items. For example,

n7:0,L122,noopt

ignores the 100 word limit on a DH+ network and reads the full 122 words. This item is sent in its own packet.