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SMS OPC I/O Server for Telemetry SCADA

SCADA systems communicating to large numbers of far flung controllers/ PLCs communicate to them using a dedicated link (copper, optic fiber or radio) or have to depend on the telecom network. Especially in cases where the telemetry system is commissioned later in the life cycle of the project, or due to various other reasons, it may not be possible to install the dedicated communication media. Telecom based connectivity for such systems is a viable option. While less critical systems and 'monitoring-only' systems can suffice with normal PSTN (analog) lines, ISDN, GSM or Leased Lines may be required for more critical and remotely-controlled applications.

The surge of District Cooling and Remote Billing systems in the Middle East has created a kind of middle market - one where the application is critical as remote monitoring and control is involved, but at the same time due to the large number of installations, it is not economically feasible to invest in expensive Leased Line or ISDN links for each and every location.

  1. A PSTN polling system would be economic to operate if the locations are within the same billing area, so that there is no additional charges related to each call. The disadvantage of such a system would be that individual out-station would need to be polled in a cyclic fashion, so any event or alarm in a location will be noticed by the SCADA only on the next poll.
  2. A GPRS based system has the advantage that all remote locations are "always connected', so any event or alarm can be reported immediately to the control room. The disadvantage of using the public GPRS network is the obvious lack of security, as the station would be accessible to all other GPRS users of the telecom provider. GPRS based leased lines allow sim cards to be on a private address, but it comes with monthly fees.

That leaves us with the common denominator in the GSM network with an always-on capability - sms text messaging. Most RTUs have the capability to generate an sms based on triggers, which could be an alarm or event. These text messages can be sent to an I/O server which decodes them and passes it as SCADA-readable data on an OPC interface. This allows any number of out-stations to communicate alarms and events as they happen, to the SCADA system. The following summarises the I/O server function.

Monitoring Function

  1. An alarm or event occurs at a remote location. It could be either a digital variable changing state or an analog variable whose value has just deviated a given % from its previously reported value.
  2. The remote RTU creates an sms based on the event and sends it to the I/O Server
  3. The I/O server validates the sms source based on the Caller Id of the sending station hashed with a shared security key. If the code matches, the I/O server decodes the sms and updates the data in its OPC interface. This data gets updated at all clients who have subscribed to this data.
  4. The data gets updated in the SCADA, which is one of the OPC clients

Control Function

  1. The SCADA operator initiates a command or setpoint change from the SCADA. The SCADA updates the value to the I/O Server on the OPC interface
  2. The I/O Server dials into the RTU GSM and transfers the changed variable using modbus. If the sms system is a backup to a polling system, it might be sufficient to set a use the polling software to update the variable on the next regular poll.

Text based SCADA Function

  1. A mobile SCADA operator logs into the I/O server by sending a login sms from his mobile phone
  2. The I/O Server acknowledges the login by sms
  3. When a new event sms is received by the I/O Server, it will be copied to the operator's mobile phone if he is subscribed to the event
  4. The operator can acknowledge alarms from his mobile phone.
  5. The operator can operate equipment from his mobile phone. The I/O server can be configured to selectively require confirmation on certain operations. Operations permitted from mobile phones will be based on safety considerations.
  6. The operator can issue an sms to logout of the the system, thereafter he will not be notified of alarms by sms

Web based SCADA Function

  1. A SCADA operator connects his Desktop/ Tablet Computer/ PDA to the ip address of the I/O server (The I/O server is available on the ip address of its host machine. Due to safety considerations, security and connectivity to wifi or vpn based internet is to be addressed separately).
  2. Using AJAX streaming technology, a SCADA login screen will be displayed. The operator can log into the I/O Server and operate his system as if he is working on his SCADA station.

We are in the process of testing the SMS OPC I/O server. We hope to put up a fully functional beta version for testing soon. It is expected that the starter version will support five events and be completely free for commercial use.