When connected to third party printers, the newer ROM-based Macintosh Plus
serial driver occasionally doesn't work, possibly because of differences
between the old RAM and ROM drivers and the newer ROM driver.
If you have recently upgraded your system to a Macintosh Plus and your printer
no longer works with some of your applications, you may have to change from
Xon/Xoff (Software) to DTR (hardware) type handshaking, if your printer has
this option.
There were originally three different serial drivers, two RAM serial drivers
and a ROM serial driver. The RAM drivers had features that the ROM driver
didn't. One version of the RAM driver was for use with the 128K and 512K
Macintosh and one version was for use with the Macintosh XL. All serial
drivers default to 9600 baud, 8 bits data, no parity, and two stop bits with
the ROM serial driver supporting CTS hardware handshake and the RAM driver
supporting Xon/Xoff software handshake. The ROM driver would terminate input
requests when an error occurred. The RAM driver would not terminate with
errors. It can be seen that, depending on which driver was used by an
application, a user could access certain built-in functions.
The newer serial driver in the Macintosh Plus 128K ROM has some differences
from all the previous drivers. The new driver verifies that the serial port is
correctly configured and free; if not, an error is returned. When opened, the
new driver defaults to hardware handshake ON (as did the old ROM driver). When
opened, the new driver also asserts the DTR line and negates that line when
closed. There have been new control calls implemented that let the application
control the input data flow from an external device, as well as set control
options, and modify the translation of parity default characters.
Further information on the old RAM and ROM serial drivers can be obtained from
Inside Macintosh Vol. II, Page 245-255. Further information on the newer ROM
serial driver can be obtained from Inside Macintosh Vol. IV, Page 225-228.