The printer ports on PowerBooks are not reliable enough for MIDI communication. Symptoms may include garbled transmissions and failure to identify MIDI interfaces connected to these ports. SerialDMA will not help in these cases. For MIDI use, Opcode recommends using the modem port (only) on these computers.
SERIAL DMA
Apple has created a new serial driver called SerialDMA that provides improved serial performance on Power Macintosh computers (and 660av and 840av Macintoshes). OMS' Modem/Printer driver, which is used by all of the MIDI drivers to access the serial ports, can now optionally utilize SerialDMA.
Enabling SerialDMA
To control whether OMS uses Serial DMA or accesses the serial hardware directly, there is a new check box in OMS Setup's Preferences dialog: Use Apple Serial DMA driver when available. If you experience MIDI communication problems, you should uncheck this option.
This preference defaults to off on the following machines: 660av, 840av, 6100.
With SerialDMA, problems with overrun errors on the printer port and general unreliability with MacOS 7.5.2 should be solved.
NOTE: SerialDMA is “built in” to System 7.5.3 (System 7.5 Update 2.0). The OMS installer won't install the SerialDMA extension on machines running System 7.5.3.
SerialDMA - OMS Compatibility
There have been problems reported with OMS using Serial DMA on the Power Macintosh 6100 and Quadra 660 av. If you encounter problems, make sure the Use Apple Serial DMA Driver when available checkbox is unchecked.
Known SerialDMA 2.0.2 Conflicts (from FreePPP docs)
Serial DMA 2.0.2 is not compatible with the extension named Serial Port Arbitrator (from ARA) on PCI PowerMacs and will cause a crash at startup. Since Serial Port Arbitrator does not show up in most Extension Managers you must manually remove it (it's not necessary for ARA to work).
The Global Village extension (sorry, I don't know the name) must load after the Serial DMA 2.0.2 extension. If you are using a Global Village modem, you should add a space to the beginning of the Serial DMA 2.0.2 Extension's file name.
Serial DMA 2.0.2 should not be used with the extension 'ASFU Fixer' which was required for the GDT Softworks PowerPrint printer adapter. Versions of the PowerPrint adapter with a small hole in the adapter plug that goes into the printer (i.e. those sold in the last 6 months or so) are compatible with Serial DMA 2.0.2. If you have an older adapter contact GDT Softworks about updates.
It seems Wacom has a similar 'fix' for their serial port connected graphics tablets. I don't know what the name of the offending extension is however. It is also unclear if Serial DMA 2.0.2 addresses the problem.
Other Possible Conflicts
If you use FreePPP 1.0.2 while running an OMS application, then do something which turns off MIDI (e.g. quit the application), your computer may freeze. This problem appears to be fixed in FreePPP 1.0.5, the latest version as of this writing.
If you use Shiva LanRover or NetSerial software, or other software which allows use of a network modem, you will need to do one of the following:
• Disable the network software when working with MIDI.
• Configure it to not intercept use of the serial port to which your MIDI devices are connected.
• Do not use the serial port it intercepts for MIDI.
