905U-G Wireless Gateway
User Manual
Page 13
©September 2004
1.2
The 905G Structure
The 905G has three functional sections:
•
The Radio Interface consists of an I/O database (or "Process Image") that maintains the
latest values of all I/O in the wireless I/O system. The I/O database comprises 4300 x 16
bit I/O registers and 4300 x 16 bit status registers. There are also other registers in the
database that can be used for system management - they are discussed later in this
manual. NOTE – the terms ‘Radio Interface’ and ‘I/O database’ are used interchangeably
throughout the manual.
•
The radio port allows the 905G to communicate with other 905G and/or 905U modules
using the 905U protocol (called “ELPRO 905U”). Messages from the 905U modules are
received by the radio port and used to update the input values in the 905G Radio Interface.
The radio port also creates the correct radio message to set outputs on the remote 905U
modules.
The ELPRO 905U protocol is an extremely efficient protocol for radio communications.
Radio messages can be sent using exception reporting - that is, when there is a change of
an input signal - or by read/write messages. Each message can comprise a single I/O
value, or multiple I/O values (termed a “block” of I/O). There are also update messages,
which are sent for integrity purposes. Messages include error checking, with the
destination address sending a return acknowledgment. Up to five attempts are made to
transmit the message if an
acknowledgment is not received.
The ELPRO 905U protocol is
designed to provide reliable radio
communications on an open
license-free radio channel.
•
The Fieldbus port enables
communications between a host
device, which could be a PLC,
DCS, HMI, intelligent transducer,
etc), and the 905G Radio Interface
database. A “host device” may be
one or several devices connected to
the same fieldbus or network (for
example, an Ethernet LAN) - in this manual, the LAN is considered as a “host device”.
The fieldbus port decodes messages from the host device and reads or writes I/O values to the
database. The fieldbus port can also generate messages to the host device.
The 905G I/O database effectively isolates the fieldbus and the radio network. This provides
a high level of system performance. The 905U radio protocol is very efficient and reliable
for radio communications. It minimizes radio channel usage by "change-of-state" reporting,
and allows the use of intermediate repeater addresses. It also allows peer-to-peer (905U to
905U, 905G to 905G) and peer-to-master (905U to 905G) communications. PLC protocols,
by comparison, are designed to provide transfer of large I/O files by "wire" link. The 905G
retains the advantage of both protocols in their respective communications media.
FIELDBUS
INTERFACE
FIELDBUS
PROFIBUS
ETHERNET
MODBUS
DF1
DEVICENET
RADIO
PORT
RADIO
INTERFACE
I/O
DATABASE
905U
RADIO
INTERFACE
905U-G
ON-BOARD I/O
EIGHT DISCRETE I/O SIGNALS
