Filter - backend communication example?

[Paul Newall]

I am working on a filter for the Kodak EPS 5250.
I can make the correct data, but printing is unreliable. I suspect that I need to add some bidirectional comms between filter and printer because the windows driver sends  a command several times to the printer before sending the data as if it is waiting for a particular response.

Can anyone point me at an existing filter that uses cupsBackChannelRead() so I can look at the source code?