IBM 6400 Operation Manual page 46

twinax connection
Power On Printer
At this point, once the cable connection
and the address settings have been
verified, you should power on the printer.
LED Indicators
The Multi-Platform Interface I/O has two
LEDs labeled SYNC and SYS (see page
75). These can be used to determine line
connection status and host
acknowledgment.
When the twinax cable is first attached to
the Smart T, the SYNC LED will light,
showing that active line connection has
been made. If the LED fails to light, the line
you are connected to may be faulty.
When the interface receives data from the
host system, the SYS LED will light
indicating a job is in progress.
32 Multi–Platform Interface Feature Operation Guide