[sane-devel] Trying to understand GL640

[Gerald Murray]

The enum is probably from a different datasheet, possibly an earlier
hardware version?

A USB control message has an 8 byte setup packet with format
   byte requesttype,
   byte request,
   word value,
   word index,
   word length,
   .. remainder of packet, if any, follows

The value shown in the enum is the value to be inserted into the "word value"
field above when sending a USB control message.
In the backend canon630u-common.c function gl640WriteControl uses
that enum value and inserts it into the field "value" and sends the USB
control message.

I did not see the USB control message discussed in that .pdf.
Do not confuse that enum and its values with the General Purpose
Registers (which actually exist in hardware) on page 10.
The USB control message is a structure of data bytes which
are in transport to/from host/device; it does not have hardware
registers.

regards,
Gerald