Problem in Modbus RTU Rs485 Communication

[veerabahu]

The details which is given below is the Polling and Response Data From My Controller.The data which is Highlighted in Bold is the
Expected data(01 03 | 06 00 0A 00 0B 00 0C). Before that Expected Data i got some Junk Data ( FF FF FF F9 F5 69). How to avoid that.



Write data

00000000: 01 03 00 00 00 03 05 CB

Read data

00000000: FF FF FF F9 F5 69 01 03 | 06 00 0A 00 0B 00 0C C8
00000010: B3

[Ttelmah]

Describe your bus layout.
Is there anything biasing the bus to 'off' when it is not driven?.
Look at:

<http://www.control.com/1026208408/index_html>

I'd suspect you are getting the 'false starts' described here.

Modbus leaves whether this is needed up to the user. Some RS485 receivers are designed so an undriven bus is seen as idle. Chips like the ISL32496 don't require any biasing to avoid false triggers, but for most chips biasing is needed. Look at the Texas note about this:

<www.ti.com/lit/an/slyt324/slyt324.pdf>

Best Wishes

[veerabahu]

Dear Ttelmah

Thanks for your reply. I have gone through both the links which you have given. I understood about false start. I included biasing resistors in A and B pin of RS485. i.e 680ohms pullup to A and 680ohms pulldown to B. But it didnt work. I'm getting the same response.

One more thing I need to clarify. For checking I have taken out my RS485 IC from circuit and i sent a polling command. For that I'm getting response as F9 FF FF FF F9 F5 69. How is it possible? Without Max IC why am I getting this value? Is it is echo problem or any other reason ? Pls give some clue.

Thanks in advance

[Ttelmah]

Without the chip, the RX line into the PIC is floating. Hence triggers....

Generally, if you are turning 'off' the receive part of the driver at any point, you must have a pull up resistor on the RX input of the PIC, otherwise noise will be seen as false triggers.

Best Wishes