Valleylab Force FX-8C, Service Manual

Page: 63 / 218

Control Board
Principles of Operation
Force FX-8C Service Manual 4-7
Feedback Microcontroller Memory
A PSD412 (U6) provides program memory (64K x 8 external EPROM) and
data memory (2K x 8 external static RAM) for the feedback
microcontroller. Additional data memory is available from these sources:
• 256K x 8 microcontroller internal RAM
• 4K x 8 external static RAM (U4) shared with the main microcontroller.
Shared RAM
The 4K x 8 external shared static RAM is provided by an IDT 713425A
device (U4) with semaphore flags. The shared RAM allows the main
microcontroller (U5) and the feedback microcontroller (U11) to share
common variables. It functions as a communications interface between the
main and feedback microcontrollers. It also provides additional general
purpose RAM to these microcontrollers.
I/0 Expansion
Three devices provide I/O expansion capabilities:
• One PSD412 programmable peripheral (U6)
• One PSD413 programmable peripheral (U3)
• One 82C55 expansion port (U2).
Each programmable peripheral device incorporates forty individually
programmable I/O pins divided into five 8-bit ports. Twenty-four of the
general I/O pins can alternatively be used as I/O for two PLDs, featuring
a total of 59 inputs, 126 product terms, and 24 macrocells. Each device also
contains EPROM (64K x 8 for the PSD412; 128K x 8 for the PSD413), 2K x 8
of static RAM, and a power management unit for battery backup. The I/O
expansion capabilities of both devices are configured as outputs for relay
control, lamp control, keyboard scanning, and chip selects.
The expansion port 82C55 (U2) is a generic I/O expander that
incorporates twenty-four I/O pins divided into three 8-bit ports. It is
configured as all inputs. It reads the keyboard, activation signals,
accessory switches, and system status flags.
Keyboard Interface and Activation Inputs
The keyboard interface is a simple row and column matrix between three
bank select output lines (BANK0–BANK2) on port A of the PSD413 (U3)
used by the main microcontroller and eight keyboard (KBD_D0–KBD_D7)
input lines on port A of the expansion port 82C55 (U2).
Port B of the expansion port 82C55 reads activation inputs from the
IsoBloc decoding circuits on the Power Supply/RF board.