The Serial Interface Module is used to program and use Federal Signal lightbars. It uses standard 12v DC wires as well as a CAT5 ethernet data cable. More information and instructions are available directly from Federal Signal here.
 
These modules are used in good condition, and include the 12v wiring harness, unless otherwise noted.
There are currently two types of this module circulating, listed here as "Old" and "New" style. They are functionally the same, but the newer model is slightly slimmer and black plastic in color, whereas the older style is silver and metal.
These modules do NOT come with a CAT5/6 (data) cable from us. Usually the data cable will be attached to the lightbar. 
A SIM module is required in order to use and program certain Federal Signal lightbars (almost all newer style LED lightbars).