GS1B-PNS-MR03 | Municipal Sewage Pump Station: Flow, Level and Pump Status Consolidated into SCADA
At a municipal sewage pump station, an electromagnetic flowmeter, ultrasonic level meters and soft starters connect to an S7-1200 through a single GS1B-PNS-MR03; the data is consolidated into SCADA via the supervisory system, and the station runs unattended over the long term.
Project Background
A pump station under a municipal sewage treatment plant needed to connect its field instrument data to the plant SCADA. The station control cabinet uses a Siemens S7-1200 series PLC, and the site is equipped with an electromagnetic flowmeter, ultrasonic level meters and soft starters, all with RS485 / Modbus RTU interfaces.
Challenges
- The three device types are located in different parts of the pump room, with the longest bus run at about 300 m
- The pump room is damp and exposed to surge risk, so the equipment must operate reliably over the long term
- The pump station is essentially unattended, so communication faults must be reported to the supervisory system promptly
Refresh Cycle Requirements for Interlock Actions
The high and low levels in the pump station are used for pump interlocking (start the pump at high level, stop it at low level), and flow and cumulative volume must also be reported to the dispatch center. The gateway the customer had been using had a long PROFINET cycle and added extra delay in its internal protocol conversion, so by the time a level change reached the PLC it was already several scan cycles late.
The GS1B-PNS-MR03 supports RT real-time communication on the PROFINET side with a 1 ms cycle, and the internal PROFINET-to-Modbus conversion cycle is likewise kept within 1 ms, so changes in level and flow reach the PLC within the next cycle and interlock actions are more timely.
Solution
The GS1B-PNS-MR03 is used, with 3 independent RS485 channels connected as follows: channel 1 carries 1 electromagnetic flowmeter, channel 2 carries 2 ultrasonic level meters (one in the collection well and one in the outlet well), and channel 3 carries 3 pump soft starters; as a PROFINET slave the gateway feeds the data to the S7-1200, which the supervisory system then consolidates into SCADA.
- Shielded twisted-pair cable with proper grounding is used for the long bus runs, routed away from power cables
- The baud rate is set to 9600 baud according to the on-site electromagnetic environment, balancing distance and stability
- The 3 channels are configured independently: servicing or replacing any one instrument does not affect data collection from the others
Key Parameters
| Item | Setting |
|---|---|
| PROFINET side | I/O slave, connected to the S7-1200 master |
| Serial side | 3 × RS485, 9600 baud, 8N1, Modbus RTU master |
| Number of slave devices | 6 (1 electromagnetic flowmeter, 2 ultrasonic level meters, 3 soft starters) |
| Topology | Shielded twisted-pair bus, longest run about 300 m, routed away from power cables |
| Power and mounting | DC 24 V (±5%), 35 mm DIN rail mounting |
Results
- Instantaneous flow, level and pump status are all displayed on the SCADA screens, so the control room can monitor the station's operating conditions remotely
- After the retrofit the station has continued to run unattended and without incident
- Independent configuration for each channel makes on-site maintenance easier