Normally, a distributed SCADA, where the information sources (RTUs) are distanced from the control center, use
protocols over leased lines or PSTN for interconnection. Examples of SCADA protocols are MODBUS and IEC
64870-5-101 (henceforth known as IEC-101).
Generally, the connection of two remote devices or RTU’s with SCADA applications is done using serial connections
such as RS-232 or TCP/IP. The REGESTA 1 allows you to configure various types of communication protocols for
its SCADA interfaces as well as different operating modes for each of them.
As an example, MODBUS uses RTUs to communicate with the central part, in devices that must be monitored.
These RTUs generally use an RS-232 compatible serial Interface, which can be connected directly or through a MO-
DEM.
Fig. 2:
SCADA Systems
MODBUS is a MASTER-SLAVE asynchronous protocol whose “MASTER” (central Host) queries the “SLAVE” re-
mote unit, identifying it with an 8-bit addressing field. The remote unit responds to each petition with the correspond-
ing answer. MODBUS defines a series specific ‘petitions’, known as functions, with the aim of acquiring data, con-
trolling and diagnosing the remote units which shape the system.
The REGESTA 1 is capable of sending information generated on SCADA interfaces over cellular telephony, encap-
sulating it in TCP/IP complying with the used protocol standard.
The following figure displays an example of SCADA connectivity (MODBUS) over a GPRS network.
Fig. 3:
SCADA connectivity over a GPRS network
Teldat S.A.
1 Description
REGESTA 1
3
