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exchange with the server. The data exchange with server is made using one of two protocols: MODBUS TCP and
modified MODBUS TCP.
5.2.2.2. Monitoring of devices connected via RS-485
The device inquires the MODBUS registers of devices connected via RS-485, upon queries from the server.
The MODBUS EM-481 registers can be specified in the server queries to read the current time, the EM-481
supply voltage, etc.
5.2.2.3. Access to MODBUS network using MODBUS TCP
EM-481 performs the function of Modbus gateway and waits for network connection via Modbus TCP protocol to
port 502. The Modbus TCP connection port can be changed by the user. Connection to PC can be made by any
programs - Modbus TCP clients. The client version for Windows software is available for download at the web-site
of NOVATEK-ELECTRO Ltd (http://novatek-electro.com/en/software.html).
At inquiry for client connection to Modbus TCP port, EM-481 checks a list of available connections. If all
connections are already engaged, the connection is cancelled otherwise the unit adds it into its internal list of
service clients (no more than specified number of clients).
When connection with a client is set EM-481 waits for Modbus-inquiry from the client. In RS-485 slave mode,
RS-485 requests from the MODBUS master are also accepted.
After receiving the query from the client, the device analyses the query and, depending on code of inquired
function and actual rights of the client, processes or blocks it. In case of blocking the query EM-481 can generate
and send to the client the specified by the user code of MODBUS exception (by default - code 1). The client's
authority level is determined by the passwords entered after connection.
If the query is address to EM-481, the device does not re-direct it, but processes it and send the reply to the client.
In RS-485 master mode, queries to other devices are redirected to the MODBUS network, and a response is
expected from the device in the MODBUS network, while the "RS-485" indicator lights up. If the data is received or
the waiting time has expired, the "RS-485" indicator goes out.
In redirection mode to a remote server, if communication with remote MODBUS TCP server is set in Ethernet
network or GSM/UMTS, queries to other devices are also sent to this server, and a response is expected from it.
Not e: The respond is received from the first responding addressee; therefore, in the MODBUS network and
among the addressees accessible via the remote MODBUS TCP server, there should not be devices with the
same MODBUS addresses (identifiers).
If the query could not be redirected (for example, in the RS-485 slave mode, if the connection to the remote
MODBUS TCP server was terminated), the EM-481 can generate and send the MODBUS user-defined exclusion
code to the client (default code is 10).
If there is no reply, EM-486 can generate and send to the client the specified by the user code of Modbus
exception (by default - code 11).
If there is a respond received on the query, EM-486 sends it to the client who had sent this query.
5.2.2.4. Access to MODBUS network using SMS
If there is GSM-connection set, EM-481 receives incoming SMS. All incoming SMS begin with a password. If the
password does not match the one specified in settings, SMS is not processed and the reply SMS is not being sent
back. After the password through the space the command is indicated.
The command consists of the following:
– the access symbol ("R" for reading, "W" for writing);
– the address (identifier) of the device in the MODBUS network;
– the resource symbol ("H" for the most frequently used MODBUS registers for storing values, while using the
MODBUS functions No.3 for reading or No.6 for writing, or "I" for the input registers, using MODBUS
function No.4 for reading);
– the address of the resource (register).
For the writing function, additionally after the space, the value to be placed at the address is indicated. For example:
SMS message "abc r1h100" will send a query to read register 100 of device 1 (if the password for
reading using SMS is specified as "abc" in the settings);
The SMS message "stanc12 w2h174 5000" is used to write the value 5000 to the register 174 of device 2
(if the password for writing using SMS is specified as "stanc12" in the settings).
If the command format is correct, the device generates a MODBUS query, which is further processed in the
same way as requests from other clients (see 5.2.2.3).
For the correct respond to the query, the EM-481 generates a response SMS. Each SMS starts with a command
accepted earlier from the user. After the command, the value of the register used in the command (both for reading and
writing) is indicated after a space. The presence of the value of the register in the SMS confirms the successful execution
of the command. For example, SMS "r1h100 2200" means that the read command from device 1 has read the value
2200 of register 100.
If the response to the query is MODBUS exception code, the EM-481 generates SMS with exception message.
SMS begins with the command received earlier from the user. After the command, the exclusion warning is
indicated after a space. The warning consists of "EXC." line and the exception number. After the warning, text
description of the exception with this number can be indicated after a space (standard exceptions to MODBUS are
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