Measurement Computing Serial488A - Manual (Page 46)

3.12
A Macintosh based Graphics System
The Macintosh computer outputs RS-422 levels. Because of this, the internal
DIP shorting jumper is set to the RS-422 position (J205).
Selecting RS-422 Signal Levels - Serial488A
J205 J206
RS-422 RS-232
Shorting Plug