ABB drive control via modbus communication with FXCPU plc

I do programming in GX developper for ABB drive model ACS 550 control via modbus communication in FXCPU PLC,I can read the Data of drive parameters and other running variables but when i go to write the data into drive for start/Stop or speed reference its not wrriten on to the drive i feel helpless now please suggest what i will do now ???

Thanks in advance

It has been a long time since I have done this, But I know you can look on the drive to see the status in bits of the drive input word. If you see all the all the correct bits turn on and the speed reference word change, then most likely one of your parameters not set correctly to control the drive via Modbus.

Did you see DickDV's comment (4th post) in this thread

http://www.plctalk.net/qanda/showthread.php?t=53829

on the need for parameter 10.01 to be set to 'comm'?

First i would like to say thanks u both for replying...........

@dginbuffalo.... mate Please tell how can i check correct status bit of drive for write the refeference value into it. Also may u knw about that perticular parameter of drive (ACS 550) to see the speed reference word change value.

@Danw.....Friend I was already read this thread..and i set the parameter as per below mention point.

1. 1001 EXT1 COMMANDS 10 (COMM) Start/Stop by field buswith Ext1
selected. 40001 bits 0…3
2. 1102 EXT1/EXT2 SEL 8 (COMM) Reference set selection by fieldbus. 40001 bit 11
3. 1103 REF1 SELECT 8 (COMM) Input reference 1 by fieldbus. 40002

But still clueless.