The external Scanning Interface Unit (SIU) has a female DB-25 rear-panel connector labelled Scanning I/O A. This connector provides auxiliary video inputs and a relay connection for use with 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].

4/17 Video B input from microscope 5/18 Video C input from microscope 6/19 Video D input from microscope 7 relay (RLY_NC) [see 2.2 relay enable circuit] 20 relay (RLY_COM) [see 2.2 relay enable circuit] 8 relay (RLY_NO) [see 2.2 relay enable circuit]

For Scanning Interface Units with the 1.1 circuit board only, the following connections apply [see 1.1 relay enable circuit]:

4/17 Video B input from microscope 5/18 Video C input from microscope 6/19 Video D input from microscope 7 relay (EXTSWT A1) 20 relay (EXTSWT A2) 8 relay (EXTSWT A3) 21 relay (EXTSWT B1) 9 relay (EXTSWT B2) 22 relay (EXTSWT B3) 10 relay (EXTSWT C1) 23 relay (EXTSWT C2) 11 relay (EXTSWT C3) 24 relay (EXTSWT D1) 12 relay (EXTSWT D2) 25 relay (EXTSWT D3)