Technical Q&A DV42

Checking for the Printer Driver

Q: On the latest Power Macintosh systems, the iBook, iMac, and PowerBook systems which have USB built-in, if I write to the Printer driver, .BOut, the system hangs. I can use Macsbug to see that there is a unit table entry for these drivers. What should I do?

A: One should never assume that the modem port drivers, .AIn and .AOut, and the printer port drivers, .BIn and .BOut, are present. The supported method to access any serial port is to first check with the Communications Toolbox Communications Resource Manager as documented in Tech Note 1119, "Serial Port Apocrypha". The Technote provides sample code on how to detect the available serial ports.

Do not use the OpenDriver call success to check for the presence of these ports. It's a known issue that on the new Power Macintosh systems with built-in USB, one can call OpenDriver successfully to try and open the .BOut port, but that an asynchronous write to the driver can cause the system to hang. The unit table drivers for the printer port exist to support USB to Serial adapters on these newer systems.