• The Model 100 Breakout Box and Cable Tester is a 100LED
  test set that accesses and monitors all 25 conductors
  of the RS-232-C, the new EIA-232-D and CCITT V.24
  Interfaces between your DTE and DCE.
• All four possible signal states (high, low, active
  or off) can be monitored using the 100 LEDs.
• A set of 25 miniature switches allow individual circuits
  to be broken and cross-patched using the stackable
  patch leads.
• RTS, CTS, DTR and DSR and a data crossover function
  can be controlled by 8 miniature stitches without
  using patch cables.
• Any 25 way cable can be tested by plugging the two
  ends into the Model 100.
• All connections and shorts or open circuits can be
  monitored by watching the LEDs.
• A remote cable test adaptor enables installed cables
  to be tested without disruption of the installation.