Baumer ETTNM User Manual

EN

INSTALLATION INSTRUCTIONS MODBUS TEDM - ETTNM

Modbus Communication - Overview

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.

Modbus communication - Principles

Master

Slave device A Slave device B Slave device C

E.g.: slave device = TEDM / ETTNM ...

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.

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

Query

Response

Frame for query/response

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 func­tion.
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 excep­tion 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

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

Query:

Device #
of slave

1 byte 1 byte n bytes 2 bytes

Function code

+ error bit

Specific information
about the command

Checksum

Response:

Device #
of slave

1 byte 1 byte n bytes 2 bytes

Function code

+ error bit

Transmitted data Checksum

Response in case of an error:

Device #
of slave

1 byte 1 byte 1 byte 2 bytes

Function code

+ error bit

Exception code Checksum

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320124 ind a 11/2012

General format of a frame

Two types of encoding may be used to communicate over a Modbus network:

ASCII - each byte making up the frame consists of 2 ASCII characters

RTU (Remote terminal unit) - each byte making up the frame consists of 2 hexa-

decimal characters (2 x 4 bits).

ASCII mode allows having gaps of more than one second between the different char­acters without causing errors, while the RTU mode allows more data to be passed at
the same transmission speed.

The TEDM and ETTNM use RTU coding to communicate over the Modbus net­work.

ASCII frame:

START Address Function Data LRC END

1 chara 2 charas 2 charas n charas 2 charas 2 charas

RTU frame:

START Address Function Data CRC 16 END

1 byte 1 byte 1 byte n bytes 2 bytes Silence

RTU Type

The maximum data size is 256 bytes. All the information contained in the message is written as hexadecimal.

The master instructs a slave. The function code tells the slave what type of action to
carry out. For example: read the register, function code 03 hex.

The data field is coded for n words in hexadecimal from 00 to FF, or for n bytes.

Depending on the function code, the data field may hold different additional informa­tion allowing the slave to interpret the message.

If RTU coding is used, the checksum CRC (Cyclical Redundancy Check) allows
the integrity of the received data to be checked. The CRC contains a value which is
encoded as 16 bits..

The slave sends its response; it places its own address in the address field so that
the master can identify it.

For a normal response, the slave uses the same function code as the one in the
message sent by the master.

The checksum field contains a value which is encoded as 16 bits. This value is the
result of a CRC calculation using the message.

Data carrier

Each byte making up a message is transmitted as follows in RTU mode:

No parity check START Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 STOP STOP

With parity check START Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Parity STOP

In the TEDM and ETTNM configuration menu it is possible to opt for parity
checking or not.

Before and after each message (frame), there must be a silence lasting a minimum
of 3.5 times the time needed to transmit a byte.

The whole message must be sent continuously. The maximum time between 2 bytes
must be less than 1.5 times the time required to transmit a byte.

Otherwise there is a transmission error.

The MODBUS protocol only defines the message structures and their exchange
mechanism.

If a parity check is used, you need to confirm the reference status: ‘even’ or ‘odd’.

Message from the

master

Minimum delay of 3.5 times the

time needed to transmit a byte

You can use any transmission interface, RS 232, RS 422 or RS 485, but the RS485
connector is the most widely used because it supports multipoints.

TEDM-ETTNM:
Data transmission speed: 9600 baud. Interface: RS485.

Message from the

slave

Minimum delay of 3.5 times the

time needed to transmit a byte

TEDM/ETTNM - Function codes

The TEDM and ETTNM use 4 function codes.

These function codes allow collection of:

- the value of the pressure reading (TEDM) or the temperature (ETTNM)

- the status of each coil (open or closed),

and writing of

- the status and setting of the coils

- the access code

- the address for the slave

Code Function Action

01 Read Coils Status Read the status of coils 1 and 2: open or closed

03 Read Holding Register Read the measured value. Pressure (TEDM) or

05 Write Single Coil

06 Write Single Register

Read the coil setting: Normally Open (NO) or
Normally Closed (NC)
Read the position of the decimal point (display)

Temperature (ETTNM)
Read the access code
Read the value of the high and low switching points
for each coil
Read the value of the timer for each coil

Write the coil’s configuration: NO or NC

Write the access code
Write the value of the high and low switching points
for each coil
Write the value of the timer for each coil
Write the slave’s address (TEDM or ETTNM)

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