Control from a Festo valve terminal

Too bad that you couldn't get it working. Those kinds of things can be very difficult to troubleshoot without a scope to check what is actually going on.

mgm wrote:

What would I need to determine more precisely where the dog is buried ?

A living dog!

Sorry, i could not resist!

The simplest you can try is to connect a second usb-rs485 adapter on a PC as a reciever and see what you get sent out on the one you are using to connect to the valve block.
There are also cheap tools like 'BusPirate' which can be helpful.
A two channel oszilloscope would be most helpful, preferably one with UART decoding.
Without any of the above it's just an endless trial and error adjusting interface parameters and swapping wires around. But there is certainly a learning curve to all of those methods.

From what I can tell looking at the screenshots you posted I still think that either your serial parameters on the PC don't match the settings in the valve block and/or there is a problem with the wiring. It hasn't been able to establish a connection to even get to the stage where it could try to initialize the device.

[edit]
And of course the cheap usb to serial adapters can be very flaky. I've had some that just simply didn't work at all.