
M-Bus Interface Module
MOTOROLA
MC68307 USER’S MANUAL
7-3
7.2.1 START Signal
When the bus is free, i.e., no master device is engaging the bus (both SCL and SDA lines
are at logical high), a master may initiate communication by sending a START signal. As
shown in
Figure 7-2, a START signal is defined as a high-to-low transition of SDA while SCL
is high. This signal denotes the beginning of a new data transfer (each data transfer may
contain several bytes of data) and wakes up all slaves.
7.2.2 Slave Address Transmission
The first byte of data transfer immediately after the START signal is the slave address
transmitted by the master. This is a seven bits long calling address followed by a R/W bit.
The R/W bit tells the slave the desired direction of data transfer.
Only the slave with a calling address that matches the one transmitted by the master
responds by sending back an acknowledge bit. This is done by pulling the SDA low at the
7.2.3 Data Transfer
Once successful slave addressing is achieved, the data transfer can proceed byte by byte
in a direction specified by the R/W bit previously sent by the calling master.
Each data byte is 8 bits long. Data may be changed only while SCL is low and must be held
stable while SCL is high as shown in
Figure 7-2. There is one clock pulse on SCL for each
data bit, the MSB being transferred first. Each data byte has to be followed by an acknowl-
Figure 7-2. M-Bus Transmission Signals
SCL
SDA
START
SIGNAL
ACK
BIT
123
4567
8
MSB
LSB
123
4567
8
MSB
LSB
STOP
SIGNAL
NO
SCL
SDA
123
456
7
8
MSB
LSB
1
2
5
678
MSB
LSB
REPEATED
34
9
AD7
AD6
AD5
AD4 AD3
AD2
AD1 R/W
XXX
D7
D6
D5
D4
D3
D2
D1
D0
CALLING ADDRESS
READ/
DATA BYTE
AD7
AD6
AD5
AD4
AD3
AD2
AD1 R/W
AD7
AD6
AD5
AD4
AD3
AD2
AD1 R/W
NEW CALLING ADDRESS
99
XX
ACK
BIT
WRITE
START
SIGNAL
START
SIGNAL
ACK
BIT
CALLING ADDRESS
READ/
WRITE
STOP
SIGNAL
NO
ACK
BIT
READ/
WRITE