The 408M is equipped with a heartbeat LED that indicates processing activity on the PCB. (The 408M peripheral processor is operating when the heartbeat LED is flashing.) The KSU1 operation LED (located next to the power switch) is tied to the 408M heartbeat LED. Therefore, when the LED next to the power switch is flashing, the 408M is active. The KSU1-408M has three ribbon cables: • located at the upper right and oriented in a horizontal position, the J1 cable is used to interface a COM4 if required to expand the system CO line capacity. • located at the upper center and oriented in a vertical position, the J4 cable is used to interface the DX-80 CPM. • located at the upper center and oriented in a vertical position, the J5 cable connects to the standard APM4 installed in KSU1. Each CO line circuit incorporates over-voltage protection, ring detector, loop detector, loop/pulse-dial relay, current sink circuit, coupling/isolation transformer (impedance 600:600), hybrid circuit, CODEC & filter, polarity guard circuit and Radio Frequency noise filter. The fourth CO line port is equipped with CNG Fax Tone Detection circuitry. When programmed as a “FAX” line, this circuit will automatically engage the FAX Tone detector. If FAX tone is detected, the system routes the call to the analog port designated as the destination for fax calls.