Project: Baton Rouge Wastewater Pump Station SCADA System (February 21,2006)
February 21, 2006
Monitoring of bandwidth usage during the past week indicates our current polling scheme is not very data efficient. We need to stay within 2 megabytes per month data usage per site or data transmission charges will soar vs. the allocated budget. Considering the 27 words of data to be transferred, one might be led to believe updates every few minutes would be possible. The reality is that to send and receive a Modbus message, there is considerable overhead. To reduce the impact of the overhead, bits are preferably packed into words, and words are kept sequential within the RTU memory map so that one Modbus function call can read a series of registers in one request. Lacking a bit packing function, we have used a sequence of if/then statements to pack bits into words. The original RTU programs were not written sequentially (and could not be written sequentially due to controller addressing requirements). Moving values around inside the controller to force data into a sequential block will allow the OPC server to read all values in one read.
The primary consultant announced last week that they intended to have a PLC and HMI recommended and selected within the next two weeks. Seeing will be believing—past history indicates this projection is probably overly optimistic.