Baumer ETTNM - Manual (Page 2)

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 informa-
tion 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..
RTU Type
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.
Each byte making up a message is transmitted as follows in RTU mode:
Data carrier
Message from the
master
Message from the
slave
Minimum delay of 3.5 times the
time needed to transmit a byte
Minimum delay of 3.5 times the
time needed to transmit a byte
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.
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
TEDM/ETTNM - Function codes
The maximum data size is 256 bytes. All the information contained in the message is written as hexadecimal.
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.
If a parity check is used, you need to confirm the reference status: ‘even’ or ‘odd’.
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.
Code Function Action
01 Read Coils Status Read the status of coils 1 and 2: open or closed
Read the coil setting: Normally Open (NO) or
Normally Closed (NC)
Read the position of the decimal point (display)
03 Read Holding Register Read the measured value. Pressure (TEDM) or
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
05 Write Single Coil Write the coil’s configuration: NO or NC
06 Write Single Register 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)
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 char-
acters 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 net-
work.
General format of a frame
START Address Function Data CRC 16 END
1 byte 1 byte 1 byte n bytes 2 bytes Silence
RTU frame:
START Address Function Data LRC END
1 chara 2 charas 2 charas n charas 2 charas 2 charas
ASCII frame:
320124 ind a 1
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