Help !! Comm Issues with VFD

Hey everyone,

Last time I was here you guys where awesome but once again I'm having problems and cant seems to figure it out !!

I'm using RSLogix Micro Developer to control an Curtiss Benshaw VFD (RSI S4)
through Modbus I can read and write to the VFD registers using the message instructions no problem however when I try to control the drive by putting it in serial mode the drive will fault(FLT:Last Trip 1 01 Comm Loss)I've double check all comm settings on the PLC and the VFD and they all match up and seem good

VFD
I/O 46 =RTU 81 (No parity, 8 data bits, 1 stop bit)
I/O 47 =14 (Comm Drop # )
I/O 48 =19200 (Com Baudrate)
I/O 49 =5.0 (Com timeout)

PLC
Channel 0
Driver = Modbus RTU
Baudrate = 19200
Stop Bits = 1
Data Bits = 8

ControlLine = No handshaking (485 Network)
InterChar. Timeout = 50
Pre transmit delay = 50

My program works my pooling the drive by using message instructions in a sequence using up counters to avoid message collision. I read all information from the drive no problem and I have them displayed on an HMI screen.
However, sending control words to drive will cause it to fault COMM LOSS??

As for configuring the drive to work in serial mode:
I have DRV mode 1 (04, 05) set to (terminal/frequency keypad reference ) (T/K) and DRV mode 2 (06, 07) (serial/frequency serial reference). but Control word Bit 14 is suppose to made mode 2 work instead of mode 1. but it will cause it to fault instead? We tried manually programming serial mode to the drive but we have the same faults

Has anyone seen similar issues?
any help or advice is appreciated!!!!!!!!!

Thanks guys !!!!

Benshaw doesn't seem to provide manuals online so this is just a guess. But I suspect that when you are trying to control the drive state over a comm network that doesn't support this natively you need to provide some type of recurring change value to indicate the link is alive. This could be a bit that toggles every so often or a value that increments every so often. But it would be a way for the drive to know command values are coming through to it.

Keith