M-Bus Master Design for more than 200 Slaves

Thread Starter

Flamador

Joined Oct 1, 2020
2
Hi. I need to read M-bus slave devices. There is 200 or maybe 250 slave devices. I want to convert this m-bus protocol to rs232 to read on MCU. How can I design this circuit ?

I find a module "M-Bus Master Click" by Mikroe, This is link "https://www.mikroe.com/m-bus-master-click". This module supply for only 10 slave devices. Can I improve this module for more than 200 devices.


mbus.PNG
 

nsaspook

Joined Aug 27, 2009
7,733
Where does it specify it's only good for 10 slave devices? I only took a quick look at your link so maybe I missed that information.
 

Papabravo

Joined Feb 24, 2006
14,672
As a general rule, you need to consider the parallel impedance of 250 receivers. Do a back of the envelope calculation with each receiver having an input impedance of 20KΩ. this is the impedance that each transmitter must drive. Long cables will compromise the ability of a transmitter to make a signal that will still allow a receiver to distinguish the different levels. I think 200-250 nodes is a complete fantasy pipe dream unless you can provide some actual evidence to the contrary. That 100 Ω resistor across the differential input of the receiver is a dead giveaway that you won't be able to make it.
 

nsaspook

Joined Aug 27, 2009
7,733
I've never used M-bus but here's another Master with 40 slave drive capability with a little more information of slave receiver drive requirements.
https://www.wago.com/us/m-bus-master-module
https://www.wago.com/medias/m075306...WE1NTVlYWQ3NzVhMTU3Yjk2OWQ2Yg&attachment=true
M-Bus standard loadmax. 40 pcs, 1.5 mA each
That gives you the standard drive requirements for X number of slaves @ 1.5 mA per slave.

There are other M-bus Masters that claim up to 250 slave capability.
http://www.adfweb.com/Home/products...Za6vl5kc5-h22gZlsKGM8ApfyY9xO5ixoCgVcQAvD_BwE
 

Papabravo

Joined Feb 24, 2006
14,672
I've never used M-bus but here's another Master with 40 slave drive capability with a little more information of slave receiver drive requirements.
https://www.wago.com/us/m-bus-master-module
https://www.wago.com/medias/m075306...WE1NTVlYWQ3NzVhMTU3Yjk2OWQ2Yg&attachment=true


That gives you the standard drive requirements for X number of slaves @ 1.5 mA per slave.

There are other M-bus Masters that claim up to 250 slave capability.
http://www.adfweb.com/Home/products/mbus_gateway.asp?frompg=GooMbus&loc_phy=9032849&k001=b&mbus-k1=+m +bus +master&d=c&pos=&gclid=CjwKCAjw_NX7BRA1EiwA2dpg0pv3OiRq1QenVGGjIrvgBFZa6vl5kc5-h22gZlsKGM8ApfyY9xO5ixoCgVcQAvD_BwE
So what am I missing about all those receivers in parallel?
Did I miss something about how they are connected together?
Neither one of the above documents was particularly helpful
 
Last edited:

nsaspook

Joined Aug 27, 2009
7,733
https://m-bus.com/documentation-wired/04-physical-layer
Transmission Characteristics

The slaves are designed to be constant current sinks with two different currents, whereby the current which is “sunk” must not vary by more than 0.2 % for 1 V voltage change on the bus. In order to transmit a Mark, a so-called Unit Load consisting of a constant current of 1.5 mA maximum is specified. If the slave needs more current, an appropriate number of additional Unit Loads must be used. When sending a Space, the slave increases its current consumption by 11-20 mA. In order to receive data, the slave detects the maximum value Vmax of the bus voltage, which can be between 21 V and 42 V. With a bus voltage of more than Vmax - 5.5 V, a Mark should be registered, and with a voltage of less than Vmax - 8,2 V, a Space should be registered.
 

Papabravo

Joined Feb 24, 2006
14,672
So the transmission is single ended with 2.7 VDC between a Mark and a Space. Does that mean that a primary device at one end of a long cable must boost the bus voltage to allow for the IR drop to the other end so, it is no more than 21 volts split between the power rail and the ground return? Do all secondary devices receive the data at the same time? What I am trying to understand is the limitation on secondary devices if it is not receiver impedance.
 

nsaspook

Joined Aug 27, 2009
7,733
So the transmission is single ended with 2.7 VDC between a Mark and a Space. Does that mean that a primary device at one end of a long cable must boost the bus voltage to allow for the IR drop to the other end so, it is no more than 21 volts split between the power rail and the ground return? Do all secondary devices receive the data at the same time? What I am trying to understand is the limitation on secondary devices if it is not receiver impedance.
The limitation seems to be master current drive capability and circuit cable loop resistance while staying within those voltage limits @ 1.5 mA per slave receiver.
https://www.ti.com/lit/ds/symlink/t...48034&ref_url=https%3A%2F%2Fwww.mouser.com%2F
https://m-bus.com/assets/downloads/MBDOC48.PDF
 

Papabravo

Joined Feb 24, 2006
14,672

nsaspook

Joined Aug 27, 2009
7,733
So this is like a throwback to the old TTY 20 mA current loop. Got it. TKS
Yea, pretty much impervious to interference. We used the old 120 vdc 60 mA High-Voltage TTY Loop for physical devices. You could string old G.I. Telephone cable for miles and still get usable marks and space at the distant end for a crypto sync at 100WPM.
cv66-rcvr-1982.jpg
kw7-museum-01.jpg

That said it's not likely the OP can boost the "M-Bus Master Click" for much more than 10 slaves without a major design of the Q2 and Q3 repeater circuit.
 
Last edited:

Papabravo

Joined Feb 24, 2006
14,672
Just one more thing. There are two connectors with two terminals each. How is the network cable wired to both connectors?
 

Papabravo

Joined Feb 24, 2006
14,672
Do the secondary devices have the same arrangement? It looks like the primary device transmits on the V+ line and receives on the data loop connector. Are the secondary devices just the reverse?
 
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