IOCTL_SERIAL_GET_COMMSTATUS not being sent

I am writing code that bypasses an old legacy application that uses rs422 to control 4 led display boxes (all connnected)
I have been using "PortMon" to check the messages going out.

Currently I get the information appearing in the first box but not the other 3 even though the chars being sent are the same.

One difference I have seen using PortMon is that the IOCTL_SERIAL_GET_COMMSTATUS is not being sent via my code. The other IOCTL code looks right although this is all happing automatically rather than being anything I have written myself.

Does anyone know why this message is not being sent?

I am using a MOXA UPort 1150 usb to serial converter.
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