KB-30910: I/O tags read algorithm.

Question:

How does the I/O tags read algorithm work for Elipse Software‘s Drivers?

Solution:

The Driver scans the I/O Tags so that the first tag to be read is the one at the top of the list, and it will be read again only after the remaining tags are read.

For each tag, the Driver checks if:

• the tag’s AllowRead property is set as True.
• the tag is in Advise (linked to an E3 object that needs its value).
• the current time is higher than the scan time plus the time of the last reading.

Example:

Consider these tags:

1. When the communication starts, all tags enabled to read (AllowRead=True) and in Advise are read.

2. If for any physical reason the driver takes 2 seconds to read the tags list, the tag scan is this:

Tag1: It needs to be updated every one second; two seconds have passed, so it will be read.
Tag2: Reading disabled.
Tag3: It needs to be updated every three seconds; only two seconds have passed, so it won’t be read.