Every Macintosh contains serial device interface ports. The ports are used
to interface to modems, to LocalTalk, to printers and a variety of other
devices. MachTen interfaces to serial ports with the support of the Unix
file system and special file names. Referencing a specific filename references
a specific serial port. Reading and writing from a special file reads and
writes data from the cooresponding serial port. These special files are
known as serial device files.
Macintosh Serial Device Interfaces
Receiving serial data is necessarily a real time operation. A Macintosh
must interrupt its computing to receive and store serial characters in real
time before more characters arrive and overrun the previous data. Until
recently, the latency between the time a character arrival and the time
Macintosh system software is able to process that data can get very large.
All Power Macintoshes and some Motorola 68000 Macintoshes contain Direct
Memory Access (DMA) hardware to remove the real time component from Macintosh
system software and support it instead by system hardware. With the advent
of Macintosh system software to properly support the DMA hardware, Power
and other Macintoshes are able to easily support serial I/O at very high
For the Macintosh-based serial device interface software, Tenon recommends
the use of Macintosh OS 7.5.3 which contains a serial DMA based driver for
use on those Macintosh hardware platforms that support DMA serial I/O. This
new software supports serial bit rates in the several hundred kilo-bit range
with extremely small system overhead. The latest version of this driver
version 2.0.2 is highly reliable and works well with all Power Macintosh
hardware and with some Motorola 68000 Macintosh systems. If DMA hardware
is not available on a Macintosh, typically Motorola 68000 Macintoshes, data
is received from a serial port on a character-by-character basis. This has
an extremely high system overhead which means that serious consideration
should be given to limiting the signalling bit rate of an modem connection
to 9.6K bits-per-second unless a very high performance Motorola 68000 system
is used in dedicated support of the serial operations.
MachTen Serial Device Files
MachTen supports access to the serial ports directly, the MachTen Macintosh
"modem" port serial device file is "/dev/ttya" and the
MachTen "printer" port serial device file is "/dev/ttyb".
These devices access the ports without regard to modem status or modem flow
control signalling. This works well for low speed operations or for character-at-a-time
operations available from programs like "tip".
When the performance of a link is improved with higher speed 14.4 or 28.8
modems, it is necessary to use hardware flow control to avoid overrunning
the modem or the Macintosh with too much data when a receiver is unprepared
to receive. MachTen supports hardware flow control for the devices "/dev/ttyfa",
"/dev/ttyfb", "/dev/ttycfa" and "/dev/ttycfb".
Ttyfa and ttyfb support hardware flow control. Ttycfa and ttycfb also support
hardware flow control and at the same time support the monitoring of the
modem carrier signal to determine whether the modem has disconnected the
To use /dev/ttycfa and /dev/ttycfb a special Macintosh-to-modem cable is
required. Many modem manufacturers ship Machintosh-to-modem interface cables
that do not connect the modem carrier or DCD signal (pin 8) to the Macintosh
GPi input (pin 7). See General Purpose Interface
(GPI) Serial Cable for more information. The GPi input receives erronous
input if the Macintosh to modem interface cable does connect this signal
and consequently will erronously descide that the modem has flagged a DCD
transition which will normally terminate MachTen PPP operations. If you
are sure that you have a cable with GPi and DCD connected use the "ttycfa"
or "ttycfb" connections. The use of "ttycfa" or "ttycfb"
if you do not have the proper cable can cause MachTen to erronously treat
a modem connection as if it had been "hung up". If you are unsure
or do not have a proper interface cable use "ttyfa" or "ttyfb".
It is important to reiterate that use of hardware flow control is essential
for modems operated speeds greater than 14.4K bps data rates.