Cannot perform a 2-sided scan using TWAIN drivers

[cdenotter]

I am writing an interface to an HP ScanJet 7000 s3 using TWAIN. Although the HP ScanJet 7000 s3 clearly supports 2-sided scanning (and I have used the feature through the standard Windows Scan and Fax dialog), I cannot get the scanner to perform a 2-sided scan using TWAIN.

After ensuring the CAP_FEEDERENABLED setting is enabled, when I attempt to query the CAP_DUPLEX or CAP_DUPLEXENABLED capabilities, I receive a  TWCC_BADCAP error. I receive the same error when I attempt to set the CAP_DUPLEXENABLED capability.

From having written interfaces to other scanners (and even other HP scanners) in the past, I've usually only had to set the CAP_FEEDERENABLED and CAP_DUPLEXENABLED capabilities to enable 2-sided scanning. Is there another capability that I need to set before duplex scanning can be enabled? Or is there something else going wrong?

[cdenotter · Accepted solution]

The issue was that I was trying to issue TWAIN commands against the WIA drivers. Once I switched to properly using the TWAIN driver (and handled some unusual TWCC_CAPBADOPERATION errors), I was able to successfully perform a 2-sided scan.