Communication (RS 485) with a Controller using ModBus RTU

[Engin61]

hi,

i'm trying to communicate with a controller (TLK31) over a RS485 ( NI PCI 8431/2 - http://www.ni.com/pdf/manuals/371253e.pdf ) port by using ModBus RTU protocol. what i want to do is to write a value in a register address.

the TLK31 has 3 wires to connect, GND, A(-) and B(+), wiring is as shown in the picture.

i try to describe the VI at the attachment:

after initializing the port i use select the number of the programmable set point (i just need one) and activate this one. after entering the address of "set point 1" (which is 0*2802) and transmitting any data value it's necessary to start the CHECKSUM calculation by writing any value at the adress 0x39B.

well, the problem is, i can't transmit any data to the device to set the value of the "set point".

thanks for any help.

[K C]

Hi,

Hardware or a software problem. Or both !!

No terminators in your picture. This could be OK if it is a short wire. But for some isolated devices you have to use terminators.

In 'my world' the A line of a RS-485 interface is the '+'. I know there manufacturers that wil reverse the '+' and '-'. In you picture the A line is the '-'.

Can you reverse the A and B connection at one side ?

Kees

[Engin61]

Hi K C,

first of all. thank you for your interest 🙂

now to answer,

i already tried the connection by inverting the pins. but the same result. no data transmitting, or any reaction from the device. the wire is about 5m long, so i guess don't need to use terminators...

[K C]

Hi,

At 5 meter I would try to add the resistors.

But before that can you try starting address 10242  ?

Kees

[K C]

In your last post you say: '..no data transmiiting..' How did you verify this ?

[Engin61]

Hi K C,

the SetPoint-Value wich i want to transmit is shown on the device-display. default value is set as 20°C. if the transmitting is successfull so the value will change into the transmitted data. 

[K C]

Ok, so it migth be that your computer is transmitting but with the wrong data. Did you try the starting address that I mentioned ?