Hi,
I'm looking into designing an M-bus master implementation. There are many details to comprehend and implement in the end, but the basics of the signaling between master and slave goes like this (my understanding):
- Bus voltage is nominal 36 Volts
- Master transmits signals by shifting bus voltage levels between 36 Volts and 24 Volts (
36 Volts=mark=1 and
24 Volts=space=0).
- Slave transmits by switching in a constant current driver of 11-20mA (
quiescent current=mark=1 and
quiescent+11-20mA=space=0)
One slave has a quiescent current of N*unit load where Nmax I believe is 4 and a unit load is 1.5mA max. So one slave can have a quiescent current of up to 6mA. If I have 2 of these slaves the bus is already pulling over 12mA, which makes me implement a way of sensing quiescent current on the bus so I know what is signaling from slave or not.
This is a quote from a pdf I found that sums it up:
The
repeater must adjust itself to the quiescent current level (Mark), and interpret an increase of
the bus current of 11-20 mA as representing a space. This can be realized with acceptable
complexity only when the mark state is defined as 36 V. This means that at any instant,
transmission is possible in only one direction - either from master to slave, or slave to master
(Half Duplex).
As a result of transmission in the master-slave direction with a voltage change of 12 V, and in
the answering direction with at least 11 mA, besides remote powering of slaves a high degree
of insensitivity to external interference has been achieved.
My question is if anyone will help me identify an okay implementation, and if maybe a circuit like this has a name already? The available chipsets are for slaves only, and the circuits examples I've seen for masters can only accommodate a fixed amount of slaves within a small range.
The document I quoted is MBDOC48.PDF from the top of this page:
http://www.m-bus.com/files/W4A21021.PDF is the most comprehensive in terms of detailed specifications, but it's listed as "Proposal for new M-Bus Standard"