December 06, 2011 | IGSS News
Scan Records and Scan Intervals and IGSS’ performance
You can increase IGSS’ communication efficiency by actively using the number IGSS’ scan records used to retrieve data from the PLC and by setting scan intervals that do not collide with each other when polling data from the connected PLC machines.
Scan Records
Be aware of the segment length of the driver you are using. Scanning for object values in data words that exceed the segment length defined for the driver will result in additional scanning and since each scanning takes time, the performance of IGSS will be negatively affected.
Example
You are using the driver number 72, which has a segment length of 110, meaning the driver can scan 110 data words in one scanning run.
You have three analog objects mapped against a Siemens PLC machine using the IGSS driver number 72 and you want to scan the state value of all three objects.
The object state values are mapped to the following addresses:
- Object 01.State: Data Block 1.Data Block Word 0
- Object 02.State: Data Block 1.Data Block Word 112
- Object 03.State: Data Block 1.Data Block Word 224
The mappings above will result in three scan records. One for the each object as the driver 72 has a segment length of 110.IGSS will only find one object value (Object 01) in the first scan from DBW0 to 110, then find one object value (Object 02) in the second scan from DBW 111 to DBW 221 and finally find the third and last object value (Object 03) in the last scan from DBW 222 to 332.
The optimal mapping of the three object could instead be as follows:
- Object 01.State: Data Block 1.Data Block Word 0
- Object 02.State: Data Block 1.Data Block Word 2
- Object 03.State: Data Block 1.Data Block Word 4
This will result in one scan record finding all three object values, saving network use, communication time and increasing overall performance of IGSS.
Scan Intervals
It can also be a good idea to focus on the scan intervals in order to improve IGSS performance. Scan intervals are set up for the IGSS configuration on the Configuration tab in the System Configuration form which is opened on the Design and Setup tab in the IGSS Master.
We recommend defining scan intervals that are not started at the same time and that are not divisible by other scan intervals.
Example
Lets say you have the default scan intervals as below:
- 2000 milliseconds
- 5000 milliseconds
- 10000 milliseconds
- 30000 milliseconds
This means:
- In the 2000 millisecond, a scan request is started for objects that are members of the 2000 scan interval.
- In the 5000 millisecond a scan request is started for objects that are members of the 2000 scan interval.
- In the 10000 millisecond, three scan requests are started for objects that are members of the 2000, 5000 and 10000 scan intervals.
- Finally, in the 30000 millisecond, four scan requests are started for objects that are members of the 2000, 5000, 10000 and 30000 scan intervals.
The last two scan intervals (10000 and 30000 milliseconds) can result in poor performance, scan lags and collisions as many scan intervals are started and PLC machines are polled nearly simultaneously. If the data has not been packed correctly on the PLC machines, resulting in slower responses to scan requests, the scan requests will be queued up and performance adversely affected.
Instead, we recommend setting scan intervals that will not result in simultaneous starts and can avoid queues and collisions, such as the following sample scan intervals:
- 3000 milliseconds
- 5000 milliseconds
- 13000 milliseconds
- 34000 milliseconds
It is also advisable to collect IGSS objects that the same scan interval in the same data area of the PLC to reduce PLC response time to the scan request.