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User Manual
M-Bus / RS232 / RS485
Document code: MN67021_ENG Revision 2.100 Page 10 of 25
M-BUS:
The M-Bus is a unpolarized bus.
A two wire standard telephone cable (JYStY N*2*0.8 mm) is used as the transmission medium for the M-Bus. The maximum distance
between a slave and the repeater is 350m; this length corresponds to a cable resistance of up 29Ω. This distance applies for the standard
configuration having Baud rates between 300 and 9600 Baud, and a maximum of 250 slaves. The maximum distance can be increased by
limiting the Baud rate and using fewer slaves, but the bus voltage in the space state must at no point in a segment fall below 12V, because
of the remote powering of the slaves. In the standard configuration the total cable length should not exceed 1000m, in order to meet the
requirement of a maximum cable capacitance of 180nF.
(Taken from M-Bus specifics)
RS232 (for HD67021):
The connection from RS232 socket to a serial port (example one from a personal computer) must be made with a Null Modem cable (a serial
cable where the pins 2 and 3 are crossed). It is recommended that the RS232C Cable not exceed 15 meters.
