KX-TDA100DUP - Panasonic PBX Service Manual (repair manual). Page 12

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LED (BATALM): Lights when the lithium battery voltage drops below 2.8V.
LED (SD ACCES): Lights on when the SD card is being accessed.
SD card slot: Mounts the system program SD card.
Reset Switch: Press to reset the system. 
System Initialize Switch:
On default startup, set the switch to the "SYSTEM INITIALIZE" position before turning the power ON. Return the switch to the
"NORMAL" position when the main power switch starts to flash. On normal startup, turn the power ON with this switch in the
"NORMAL" position.
USB connector: B-type connector. Used for PC programming.
RJ45 connector : MOH 1, 2: External music on hold input.

    PAGING 1, 2: External PAGING output. Connect to a device with a built-in amplifier.
    connected to DPT

RS-232C connector : For PC-programming or SMDR (connected to PC or a printer.)

• CPU block 

Operates the main unit control. Also operates various controls, generation of select signal, DMA control and serial port control.
This contains the built-in clock function.

• ASIC block

Provides the communication between each optional service card, call control (TSW function), conference call, tone generation
and gain control function.

• Memory block

This is a work area used for the main unit control program storage, the system boot program storage, or the user configuration
data storage.

• USB block 

Provides the USB I/F function. Connects to the PC to be used for PC programming or system data load/store.

• SD card block

Provides the SD card I/F function and loads the main unit program and the system data from the SD card containing the main
unit control program.

• MOH/PAGING block

Provides the external music on hold input x 2, and the external paging output x 2 port. Also this provides the external music on
hold input 1 system and exclusively the internal music on hold output.

• Power block

Consists of DC/DC converter circuit and various regulators.

• RS-232c block

"Content of the description of USB block" +. "SMDR function"

• DPT block 

This provides four extension ports for DPT.