The external Scanning Interface Unit (SIU) has a female DB-25 rear-panel connector labelled Scanning I/O B. This connector provides almost all of the interface signals for scanning control of the microscope. Shown below is the schematic of this connector and the master pin-out table for it. Each signal line has its own ground reference. In the table, the pins are listed as [signal/ground].

1/14 External Enable input from microscope 2/15 External Enable output to microscope 3/16 Horizontal Scan output to microscope 4/17 Vertical Scan output to microscope 5/18 Video A input from microscope 6/19 Video A output to microscope 7/20 Horizontal Blank output to microscope 8/21 Vertical Blank output to microscope 9/22 Horizontal Blank input from microscope 10/23 Vertical Blank input from microscope 11/24 Horizontal Scan input from microscope 12/25 Vertical Scan input from microscope