Commercial In Confidence
12
1.17
MODES OF OPERATION & PROTOCOL HANDLING:
1.17.1
Radio Modem Modes of Operation
The basic modes of operation of the radio modem are as follows:
1.17.1.1
Dumb modem
The radio has no knowledge of the data it is transmitting, data is simply transmitted and
received under hardware control with the option of RTS control or initiation of transmit after
receipt of serial data, with CTS providing an optional flow control.
This configuration is useful when expanding older systems where the radios must be
compatible with others of a different manufacture.
1.17.1.2
Protocol specific modem
The radio recognises a complete frame and only transmits and receives data conforming to
that format. No addressing of radios or routing of data is performed. Protocols such as
MODBUS & DNP3 can be supported in this way.
1.17.1.3
Routing modem
The radios recognise a protocol specific frame and the address to which the frame is to be
sent. Routing information must be stored in each radio for each destination address that
requires the use of repeaters. Any radio in the system can operate as a repeater. The radio
does not perform any acknowledgement or retries. Any protocol using a fixed address field
such as MODBUS can be supported.
1.17.1.4
Dial up modem
Hayes protocol is used to dial up the radio link which may include repeaters or store &
forward stations, the route information is not stored but is passed in the dial up command in
the form of a telephone number, once the link is established it is transparent and so
independent of the protocol being transported. This allows point to point protocols such as
SLIP and PPP (and hence TCP/IP) to be conveyed. Dial up is less efficient for small data
transactions because of the data
exchanges carried out during the connect and disconnect
phases.
1.17.2
I.O
.
Modes of Operation
1.17.2.1
Isolated network with point to point I/O mapping
Inputs and outputs at outstations are mapped to corresponding outputs and inputs at the
master.
1.17.2.2
Network with retrieved data access at base station.
Instead of mapping data to physical inputs and outputs at the master, data is exchanged in
memory. The memory is accessible using MODBUS. The base station carries out its data
retrieval process independently of the MODBUS accesses.
1.17.2.3
Externally controlled network
In this mode the base station only carries out data retrieval when requested to do so by the
MODBUS interface.
The above modes are not independent processes but are run according to set up, it is possible
to configure operation to be a mix of all three. E.g. some physical I/O might be desirable at
the base station whilst the rest is passed by MODBUS, the base station can be set to keep
polling independently in order to maintain the physical I/O but can also mix in commands
passed by MODBUS.
