Baumer ETTNM - Manual

The TEDM pressostat and the ETTNM thermostat have a serial RS485 port and use
the Modbus RTU communication protocol.
The Modbus protocol is a dialog protocol based on a hierarchical structure consisting
of a master and several slave devices. It supports reading the pressure level (TEDM)
or the temperature (ETTNM), as well as the status of each coil (open or closed).
Communication from master to 1 slave: the master sends a query and waits for a
response.
Communication from master to all slave devices: the master sends a message to all
slaves currently on the network, they execute the command in the message without
sending a reply.
Two slave devices cannot talk to each other.
Addressing
The devices on the bus are identified by addresses assigned by the user. The address
of each subscriber is not related to its physical location.
Master
Modbus Communication - Overview
Modbus communication - Principles
Communication from master to a single slave
The master queries the slave over the network and waits for a response from that
slave.
Master
Slave device A Slave device B Slave device C
Response
Query
Communication from master to all slaves
Communication from the master to all slave devices: the master sends a message
to all slaves currently on the network, they execute the command in the message
without sending a reply.
Master
Slave device A Slave device B Slave device C
Command
E.g.: slave device = TEDM / ETTNM ...
Slave device A Slave device B Slave device C
These addresses range from 1 to 247 and do not have to be assigned sequentially.
There cannot be two slave devices with the same address.
Factory default address TEDM / ETTNM: 246.
The query
It contains a function code telling the slave being queried what type of action is
required.
The data contains any additional information the slave may need to execute the func-
tion.
The checksum allows the slave to check the integrity of the query content.
The response
If an error occurs, the function code is changed to indicate that the reply is an excep-
tion response (MSB*=0: no error; MSB=1: error).
The data then includes a code (exception code) indicating the type of error.
Exception code:
01 Illegal function (function code error)
02 Error in the register address or coil address
08 Transmission error (after checking the CRC or the synchronization)
*MSB : Most Significant Bit
Frame for query/response
Device #
of slave
Function code
+ error bit
Specific information
about the command
Checksum
1 byte 1 byte n bytes 2 bytes
Device #
of slave
Function code
+ error bit
Transmitted data Checksum
1 byte 1 byte n bytes 2 bytes
Response in case of an error:
Device #
of slave
Function code
+ error bit
Exception code Checksum
1 byte 1 byte 1 byte 2 bytes
Response:
Query:
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1/2012
INSTALLATION INSTRUCTIONS
MODBUS TEDM - ETTNM
EN