Newbie need help in Modbus


Thread Starter


Hi everyone,

I am currently thinking of how to read a modbus message from a modbus table given. It was a MGE Upsilon modbus table and given:
(source 1 input switch) word = 1002 bit = 3

What does it means? What modbus address should i assign to get the particular status?

You need to look up the information in the manual for the feature you want to use. The information you have does not describe the address adequately.

You said: "word = 1002 bit = 3". A "word" would imply a register, but registers can be either input registers or holding registers.

What little information I have seen about this hardware seems to show everything as being in holding registers.
As M Griffin notes, documentation is critical. If the original poster had provided a link to the relevant document, there might have been more timely responses. I doubt any of us forum participants relish googling to find other people's .pdf documents on the web. But, it's Saturday and it's raining, so I did some digging.

A manual for an MGE UPS Systems serial card and its Modbus functions is here:

page 29:
3.4 In Upsilon STS
Status table
In addition to the tables of data (states and measures) available for reading, there is a table for writing commands.

If you need more information on this later table, in order to control the Upsilon STS through its JBUS/MODBUS communication, you should contact the software support team (contact available on our Web site www.mgeups

[The table below has column entries separated by dashes since the forum's fomatting never holds column spacing. The column titles are in the first line]

Status description - Status to 0 - Status to 1 - Word - Bit

Source 1 input switch (Q1) - open - closed - 1002 - 3

Source 2 input switch (Q2) - open - closed - 1002 - 4

Output switch (Q3) - open - closed - 1002 - 5

Source 1 bypass switch (Q1BP) - open - closed - 1002 - 6
Source 2 bypass switch (Q2BP) - open - closed - 1002 - 7
Source 1 active - no - yes - 1002 - 8

page 9:
The standard JBUS/MODBUS functions provided by the card are:
- function 3 : read n words,
- function 5 : write 1 bit,
- function 6 : write 1 word,
- function 16 : write n words.

I would infer from the description of the table on page 29 that word 1002 is a bit packed word where each bit represents some binary function as described in the table.

Modbus command code 6 can write a bit packed word to 1002, where
- bit 3 is the source 1 input switch status (Q1)
- bit 4 is the Source 2 input switch status (Q2)
- bit 5 Output switch (Q3)