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7. GPRS connection
7.1 What is GPRS and Modbus TCP?
GPRS (General Packet Radio Service) is a wireless, "always on" connection that remains active as long as the CIU 250 is within range of
the service. With GPRS it is possible to establish a wireless connection to the internet and thus enable a remote connection to a SCADA
system computer or another PC application. Typical data rates are 32 to 48 kbit/s.
The GPRS itself takes care of the wireless data transfer via the GSM network. It plays the same role as Ethernet in a wired network. On
top of GPRS is the TCP/IP protocol, which enables easy integration with the internet. The Modbus TCP protocol is used on the application
layer communicating with a TCP port number (default 502). The difference when compared to the fieldbus protocol Modbus RTU is the
exclusion of the 16-bit CRC checksum and the adding of a Modbus application program header as illustrated in fig.
.
Fig. 8
Modbus TCP telegram
7.2 Subscription
The GSM service providers have different technical solutions for GPRS to choose from. You have to select the service provider and the
technical solution that best suit your system, and it must be based on static IP addressing. You will get the following from the GSM service
provider:
• A Subscriber Identity Module (SIM card).
• An Access Point Name (APN), e.g. "internet".
• A username (is fixed and cannot be changed by the user).
• A password (is fixed and cannot be changed by the user).
• A static IP address.
Solutions based on a VPN (Virtual Private Network) involve the use of special routers, e.g. GRE routers, Generic Routing Encapsulation,
which you will also get from the service provider.
7.3 Installation
To prepare the CIU 250 for GPRS communication, some settings have to be made via SMS commands:
• Select Access Point Name:
APN <ascii string>
This is always mandatory.
• Select Username:
USERNAME <ascii string>
The need for a username depends on your operator and the type of subscription.
• Select Password:
PASSWORD <ascii string>
The need for a password depends on your operator and the type of subscription.
Some advanced GPRS settings have default values that usually work, but in special cases, it might be necessary to change some of
them. This is also done via SMS commands.
• Select Authentication:
AUTHENTICATION <NORMAL | SECURE>
Only used by some service providers. Default value is "Normal".
• Select Connection type:
CONNECTION <SERVER | CLIENT | DISABLE>
Default value is "Server".
• Set GPRS roaming:
GPRSROAMING: <ON | OFF>
Default value is "Off".
• Select Modbus TCP port number:
MODBUSPORT <port no.>
Default value is 502.
• Select GENIpro port number:
GENIPROPORT <port no.>
Default value is 49152. This is only relevant when using Grundfos PC Tools.
It is possible to configure the GPRS connection with a single multi-parameter command:
• SETGPRS <parameter 1, parameter 2, parameter 3, …>
– <parameters>:<APN>,<Modbus port>,<GENIpro port>,<user name>,<password>,<authentication>,<connection>,<GPRS roaming>
TM
04
4
907
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09
Modbus RTU telegram
Modbus TCP/IP data
Modbus application program header
Modbus TCP/IP application data unit
Slave ID
Func
code ID
Data
CRC
Data
Func
code ID
Unit ID
Length
Protocol ID
Transaction ID