Janitza UMG96RM RS485 Bus

3 posts in this topic

Dear All.



i'm a new user for Janitza. and i have issue related to RS485 actually, not to Janitza itself.

i have one device (UMG 96RM), where there is only RS485 (2 pin only), no RS232, no USB, No Ethernet port. i'm trying simply to connect the device to the PC, via (USB to RS485) convertor, but i'm not able to do it. for me, this is the first time i'm see (RS485 2 Pins), usually there is Ground signal also.

anyway, could you please somebody help how to connect? shall i add a pull up & pull down resistors at the device?

and what is the power supply i must use?

noting that, i tried many things, but still nothing happened.


Appreciate if somebody can help


Best regards.


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What you've got

Page 9 in the manual shows that the RS-485 port uses the Modbus RTU protocol.   RS-485 defines the electrical characteristics of the bus, but Modbus defines the rules of communications: who talks, who listens, timing, what the format of the messages is, error checks and what the error handling is.

Nothing happens

Nothing should happen when you connect your Janitza to your PC via RS-485.

Modbus is a master/slave protocol.  In Modbus, slave/servers never speak unless they are asked for information or acknowledge a message sending them data.  The Janitza box is a Modbus RTU slave.   It is sitting there waiting for a command from a Master/client.   You haven't sent it a command.   So it is silent.  You need to obtain a Modbus Master application program for your PLC and then program the master application to ask the Janitza slave for the data you need.   Modbus is a project.   There is nothing plug-and-play about Modbus.


The power used by the RS-485 communications port is derived from the power to the Janitza itself.

Signal Ground

The implementation of RS-485 is all over the map.   RS-485 should have a signal ground wire - Profibus DP is RS-485 and since it was an engineered implementation of RS-485 it has not only the 3rd wire but bias (driver lines connected to Vcc and gnd) at the master and selectable termination.  But the companies that implement RS-485 for Modbus sometimes do a pretty poor job of implementing either RS-485 or Modbus.   In the absence of a signal ground the 485 circuit 'finds' a ground somewhere to reference.  When things are good, that ground potential is close to the true signal ground. When things are bad, the ground potential between the master and various slaves exceeds the common mode voltage limitations of RS-485 and the bus communications ceases.  The typical work-around is to use RS-485 repeater-isolator modules to break the ground loop created by excessive common mode.

Grounding the shield on the comm cable

Good idea to use quality communications grade cable with twisted pair(s) and to ground the shield at one end.

RS-485 connections

Driver line A is supposed to connect to A, B to B, (+) to (+), (-) to (-), depending on how the driver lines are labeled.  But some vendors label the lines the opposite of others (some divisions in large companies that never talk to each other label the 485 driver lines the opposite of the other's).   So when you've got your Modbus RTU master programmed and it can't get 485 to communicate to the slave, one of the first things to try is to swap the driver lines on one end to see if that's the issue. 

On the bench at baud rates below 28K baud, terminating resistors generally aren't needed.  I install them in the field, but termination becomes more critical at higher baud rates.

serial settings
S-485 is a serial bus.  The serial settings for both devices MUST be identical - baud rate, 1 start bit, 8 data bits, and parity.  Check and recheck serial settings.  Most devices do not recognize serial setting changes until you cycle the power to the device, so cycle the power and the check again.

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