This was done using a 9-pin outlet on one computer connected to another computer through a null modem: a special, RS-232 serial cable that enabled communication programs to transfer files and data ...

Both of these have a hd-15 like connector except that it has only 9 pins. What kind of connector is that? The 17GLs has 5 bnc connectors, which work fine, but the 15GL just has the 9 pin connection.