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3 • Theory Of Operation
3.6 Electronics
PCBs (cont'd)
It also provides the interface to the autofeeder for the DM400c system. In
addition, it supports the keypad and display functions through an interface
to the control panel PCB. The board supports all USB communications, the
serial interface to the integrated scale, the analog modem and the feeder
communications channel. Also resident on this board are the circuit ele-
ments of the LFSR/PSOC system that encrypt data communication be-
tween the iButton/PSD and the Carriage PCB. Finally, the board develops
various local power sources (e.g., 3.3V, 1.5V) derived from the 5V output of
the power supply unit.
As shown in Figure 3-35, the PSD (meter) resides on the main logic board.
The PSD is the vault for postal funds, and debits these prior to issuing post-
age and credits upon user-selected refills from the Pitney Bowes Data Cen-
ter. The PSD includes a cryptographic engine that ensures security of the
funds transactions. Physical and environmental security measures reduce
the risk of tampering. PSD1 refers to the iButton meter; PSD2 is reserved
for future non-iButton PSDs.
The board has two status LEDs as shown in Figure 3-35. LED 1701 flashes
fast and indicates the presence of 5V at the Carriage Board which is sealed in
the print carriage assembly. (See figure 3-38 for the location of this board.) LED
1702 is the "heartbeat" lamp and flashes once per second to indicate that 5V
are present on the main logic board and that the microprocessor is running.
The DM300c and DM400c use the same main logic board, but each meter
PCN (G900, G903, G905) will have a unique part number. This is due to
the fact that the main board must have the correct PCN-specific application
code on it prior to installation.
DM300c/DM400c/DM475 Service Manual (SV61801 Rev. D)


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