Starlight Xpress SXV USB2.0 Interface - Manual (Page 5)

Getting a good result from an autoguider will often entail a lot of detective work to
eliminate the sources of gear error, telescope flexure, mirror shift etc., but the final
result is well worth the effort!
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Using the built-in serial ports
The USB2 interface incorporates two fast serial ports for use with external accessories.
The ports are available on 5 pins of the 18 way connector that is provided for the
autoguider and may be accessed by plugging in a ‘serial port divider box’. The divider
box and cables are available as an accessory and may be chained in series with the
autoguider cable, when the guider is in use, or may be used on its own.
The two serial connections are in the form of standard RS232 PC style plugs and provide
TX, RX and Ground connections at RS232 levels. Access is via commands sent through
the USB connection and, at the time of writing, is limited to any serial controls that are
provided by the SXV software. It is expected that many more functions will be added as
the software is upgraded.
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Using two, or more, USB cameras on the same computer
The original USB1.1 interface used an uploaded hex file (05472131.hex) to activate the
camera driver. Unfortunately, the hex file was not universal and it was difficult to make
the computer recognize the difference between two cameras on the same machine. This
problem has been alleviated with the introduction of the USB2 interface, as the hex file is
now resident in the USB hardware and has been allocated a unique VID & PID identifier.
This identifier can be recognized by the software and used to distinguish between the
interface and other USB devices. The interface uses a VID and PID of 12780200, the
number 1278 being the official Starlight Xpress VID and 0200 the number of the device.
So long as the software is written to recognize the VID and PID numbers, it can
distinguish between the USB2 interface and another SX USB camera.
In some few cases, two interfaces may be required to operate simultaneously on one
machine (i.e. when using an MX5 to guide an MX7). In this case it is still possible for the
software to identify the interfaces as different devices, as the camera firmware is not
being uploaded from the computer and so there is no confusion about the identity of each
camera.
Terry Platt
July 2004