Modbus Protocol Communication using RS 485

[nnarra]

Hi all,

I am facing one problem while communicating with Radix Instrument using Modbus RS 485 Interface.can anybody help me in this,

The instrument which i am going to communicate is Radix ISOSCAN(16 Channel Temperature Scanner).

The instrument is having Modbus RS 485 interface facility.

To that instrument,RS 485 Configuration is there.And I am using RS 485 to RS 232 Converter for communicating with my PC.

I downloaded Modbus VI Library from NI and I programmed using Modbus VIs.

I got the instrument Register Addresses also.From this instrument,I want to read 16 channel temperature values.For this,in Modbus terminology we will call as Process Variables(PV).

For this,I used Read Input Registers(Function Code 04) and I have doubt in configuring Starting Address terminal of "MB Serial Master Query.vi".Because,here if we see the datatype of this terminal,it is U16.But if we see the Instrument register addresses,it is of floating point type.

Actually first of all I am not getting any out put from "Registers" terminal of "Modbus Master Serial Query.vi"

Actually as i have to read(Read Only) 16 channel Data from the instrument,and as per the link mentioned in ni "Introduction to Modbus" topic,

http://zone.ni.com/d...a/tut/p/id/7675

I selected "Read Input Registers" with Function Code "04".

As per the link above,If starting address is 40008,we have to give 7,If it is 40009,we have to give 8 ...

So as per the instrument register addresses,what i got from the instrument design people,Addresses are starting from 40001,

So I tried in the following ways

Test 1:
Following as inputs for "Modbus Master Query.vi",

1.With Read Input Registers
2.With Starting address as "0",

Test 2:
Following as inputs for "Modbus Master Query.vi",

1.With Read Holding Registers
2.With Starting address as 1 also,

But in both the cases I am not getting any value from the Registers terminal of "Modbus Master Query.vi"

Also i have gone through the link which is in ni developer zone forums,i.e.
http://forums.ni.com...uireLogin=False

Also i observed one thing while running the code anna,

I am using RS 485 to RS 232 converter for communicating with the instrument.

From this converter,there are two output terminals(TXD) and (RXD) with boolean indicators(one for the TXD and one for RXD).I think these are send and receive commands for serial communication(RS 232)

Observation is that,whenever i am running the code,the boolean which correspond to TXD is blinking,

But there is no indication from boolean of RXD.

I think,the code which is running in my PC is sending command but because of some reasons there is no reply(as there is no indication from RXD boolean) from the instrument.

I am thinking that the with the starting address what i am giving in the VI,its not able to search for the registers...,


Regarding RS 485 to RS 232 Converter,

In front end,one side, two terminals(D+ and D-) are connected to Instrument D+ and D- Terminals.
Another side,three terminals(230 VAC,Neutral and Earth) given supply Line,Neutral and Earth Terminals.

And in front end,9 Pin Female(RS 232) Connector is there and i used one Serial Female to Male cable for connecting my PC to the Converter RS 232 port.

Can anybody support me so that i can solve this problem...,

Regards,

Bujjin

[CharlesD]

If you have a 32bit Floating point number (Single Precision) in your register than you must read to U16 number, concanate them by using the "join Numbers.vi and then typecast them into a Single Precisio. See the image Below for more detailed information. 

I'm not sure if you are still having problems with this if you are please re post and we will be glad to help you 

thanks