BECKHOFF DK9322-0810-0036 - Manual (Page 4)

TwinCAT
TwinCAT, Building Automation
Application Note DK9322-0810-0036
0
500
(1640)
1000
(3280)
1500
(4921)
2000
(6561)
2500
(8202)
300
0
(9842)
3500
(11483)
4000
(13123)
450
0
(14763)
total cable length [m]
(ft)
0
50
100
150
200
250
300
M-Bus devices
Number of M-Bus devices depending on the cable length
2 x 2 x 0.6 mm² (2 x 2 x #19 AWG)
2 x 2 x 0.8 mm² (2 x 2 x #18 AWG)
3 x 1.5 mm² (3 x #16 AWG)
Fig. 2 Number of M-Bus devices
5. Communication
M-Bus is a single-master bus based on serial data transmission. The master queries the meters via the bus by modulating
the supply voltage: A logical 1 corresponds to a signal level of 36 V DC, a logical 0 is represented by 24 V DC. This ensures
that the power supply for the slaves can take place via the bus. The slave responds to the master by modulating its current
consumption: 1.5 mA corresponds to logical 1, logical 0 is detected between 11 and 20 mA. Data transfer rates of 300 to
9,600 baud are possible, in some cases more than 9,600 baud. The maximum possible transfer rate of a meter depends on the
manufacturer. The user must therefore check which baud rate can be used for all connected field devices within a strand. In
delivery state (unparameterized) many slaves are set to 2,400 baud as standard.
Two addressing modes are possible:
With primary addressing the master sends a request for data transmission to the end device 1...250. The addressed slave
responds with a standard dataset, which in the simplest case consists of the meter reading, measured medium, device type,
serial number and manufacturer‘s code.
Secondary addressing can be used to extend the address space of M-Bus. With secondary addressing the master sends a
request for data transmission to all end devices via the special address 253. Individual end devices are addressed via serial
number, manufacturer‘s code and medium. The addressed slave responds with the standard dataset.
New Automation Technology Beckhoff 4
For application notes see disclaimer on the last page