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state of bit 3 (vsync) of the V1A1 chip's data register A determines which transfer
method is used. If the bit is low, the serial port is used in asynchronous mode. If it
is high, synchronous transfers are possible. You need special software which handles
this bit. The standard drivers of the Communication Toolbox (part of Apple's sys-
tem software) do not support this feature.
In many of the low-end models (i.e., Classic, Classic II, LC and LC II), pin 7 is not
connected. These models do not offer synchronous transfer.
According to Apple, loss of data is possible in certain models when the serial port
speed is set to more than 2400 bps and Appletalk is active at the same time. With
the newer models you should never encounter any problems, even when you run
speeds of 9600 bps or above.
You should not lock the serial port at more than 19200 bps if you do not dedicate
your Macintosh computer to data transfer. However, if you own a Quadra or a
Mac IIfx, you may be able to use multi-tasking at even higher speeds on the serial
port.
A number of NuBUS cards are available which provide serial capabilities with all of
the above restrictions removed.
Cable
The following table shows the wiring of a modem with a Macintosh hardware
handshaking cable:
*. With the cable described above, no DTR signal is available. To use this signal,
you need a special cable with the appropriate software for your Macintosh.
With the described cable, the modem must ignore the Data Terminal Ready sig-
nal. Set AT&D0.
Mac
Modem Description
*
Din 8
DB 25
1 4 & 20 DTR to RTS and DTR, Hardware Handshaking Out
2 5 CTS to CTS, Hardware Handshaking In
3 2 TX- to TXD, Data sent
4 7 GND, Ground
5 3 RX- to RXD, Data received
6 - not connected
7 8 GPI to CD, Carrier detected
8 7 GND, Ground, needed for RS-232 connections