2.4. Serial SCADA protocols
Even when the SCADA devices are connected via serial port, communication remains secured and
address-based in all directions (centre-RTU, RTU-centre, RTU-RTU).
In router mode, RipEX utilises a unique implementation of various SCADA protocols (Modbus, IEC101,
DNP3, Comli, RP570, C24, DF1, Profibus). In this implementation SCADA protocol addresses are
mapped to RipEX addresses and individual packets are transmitted as acknowledged unicasts. Polled
remote units respond to the unit that contacted them (multi master network possible) using secure
packets. When needed, RTU-RTU parallel communication is also possible.
2.4.1. Detailed Description
Each SCADA protocol, such as Modbus, DNP3, IEC101, DF1, etc., has its own unique message format,
and more importantly, its unique way of addressing remote units. The basic task for protocol utility is
to check whether a received frame is in the correct protocol format and uncorrupted. Most of the SCADA
protocols use some type of error detection codes (Checksum, CRC, LRC, BCC, etc.) for data integrity
control, so RipEX calculates this code and check it with the received one.
RipEX radio network works in IP environment, so the basic task for the protocol interface utility is to
convert SCADA serial packets to UDP datagrams. Address translation settings are used to define the
destination IP address and UDP port. Then these UDP datagrams are sent to RipEX router, processed
and typically forwarded as unicasts over the radio channel to their destination. If the gateway defined
in the routing table belongs to the ethernet LAN, UDP datagrams are rather forwarded to the ethernet
interface. After reaching the gateway (typically a RipEX router), the datagram is again forwarded ac-
cording to the routing table.
Above that, RipEX is can to handle even broadcast packets from serial SCADA protocols. When
broadcasts are enabled in the respective Protocol settings, the defined packets are treated as broadcast
(e.g. they are not acknowledged on Radio channel). On the Repeater station, it is possible to set
whether broadcast packets shall be repeated or not.
Note: UDP datagrams can be acknowledged on the radio channel (ACK parameter of router mode)
but they are not acknowledged on the ethernet channel.
When a UDP datagram reaches its final IP destination, it should be in a RipEX router again (either its
ETH or radio interface). It is processed further according its UDP port. Either it is delivered to COM1(2)
port daemon, where the datagram is decapsulated and the data received on serial interface of the
source unit is forwarded to COM1(2), or the UDP port is that of a Terminal server or any other special
protocol daemon on Ethernet like Modbus TCP etc. Then the datagram is processed by that daemon
accordingly to the respective settings.
RipEX uses a unique, sophisticated protocol on the radio channel. It guaranties data integrity even
under heavy interference or weak signal conditions due to the 32 bit CRC used, minimises the likelihood
of a collision and retransmits frames when collision happens, etc. These features allow for the most
efficient SCADA application arrangements to be used, e.g. multi-master polling and/or spontaneous
communication from remote units and/or parallel communication between remote units, etc.
Note: The anti-collision protocol feature is available only in the router mode. The bridge mode is suitable
for simple Master-Slave arrangements with polling-type application protocol.
RipEX Radio modem & Router – © RACOM s.r.o.
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RipEX in detail
