KX-TDA620BX (serv.man8) - Panasonic PBX Service Manual Supplement (repair manual). Page 12

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The TDA620 is a port expansion unit for the TDA600. 
Control of the telephone operates as a PBX, control of the call path, and control of the power supply of the option card contained
in the shelf are done by the BUS-S card.
BUS-S, the option card, and power supply are connected each other through the backboard, and are connected to BUS-M con-
tained in the TDA600 with a bus cable.
The LPR card has a sub-processor, communicates with the CPU of BUS-S through DPRAM of the on board ASIC and controls
various terminals.

This card is a control card for the expansion shelf. It controls the option card contained in the expansion shelf according to the
instructions of EMPR. 
On the BUS-S card are LEDs in two colors that indicate the status of the card and the bus cable connector for connecting to the
BUS-M card. 
Card component outline: 
• 32bit CPU
• SDRAMs, SRAM
•  FLASH memory
•System TSW, Communication IC

• Lithium Battery for back-up of static RAM.

BB is the motherboard of the expansion shelf.

This board connects the power supply to BUS-S and the option card.

This unit is switching regulator power supply and supplies DC voltages to BUS-S card, and optional service card (free slot). PSU
(power supply unit) has four outputs (40V, 30V, 15VPT, 15V). 15V output is supplied only in BUS-S card and option card. Other

outputs are mainly used as an object for the electric supply to a terminal. Refer to each S/M for the details of PSU.