If you want to use a Macintosh from a Rolm phone system and access a modem pool
on the switch, you need to provide DTR from the Macintosh. The standard cables
do not provide this. You need to make a special cable with the following
pinouts to provide the appropriate signalling:
DB-25 to DB-9
----- ----
1 8
2 5
3 9
7 3
20 6
8 7
Use a standard DB-9-to-DIN-8 cable to connect to the Macintosh.