6500-CX Hearing Aid Test System - Frye Electronics

4.6.14.2 Chip Selects
Two signals are used for chip selects. PSEN\ and PSENRAM\ select PROM and
RAM, respectively. PSEN\ comes directly from the printer CPU. PSENRAM\ is
derived from signals at the input of the PAL, U20; these signals include PSEN\.
The RAM chip can only be accessed when the Slave65 FF U17B is reset by a
CLRSLAVE\ signal from the PAL.
4.6.14.3 Software Control
The printer CPU accesses the circuits on the board and also the digital data
being sent from the host instrument bus. When print information is being
received, Slave65 FF U17B is set, which allows the printer CPU to access the
information in registers U16 and/or U18 which received the print information.
4.7 6500 Color Video Board (061-0121-XX)
4.7.1 General
This circuit board has been designed around the requirements of the VGA video
monitor used for IBM and IBM Clone Personal computers. It uses a 82C450 video
chip built by Chips and Technology.
The video circuit is also configured to only present this mode, and thus does not
have the usual switching circuits employed in the usual VGA video board.
4.7.2 Oscillator
A single frequency of 50.35 MHz is used for both dot and memory clocks. The
oscillator package is either an eight pin or 14 pin type with a CMOS output, and
supplies power to CLK0 and CLK1 of the the 82C450.
The video chip has built in dividers to set the internal frequencies.
4.7.3 Video Chip Addressing
The first eight address lines of the 82C450 are time shared with an eight bit data
bus. The remainder address lines are directly connected to the chip.
Address line 0 is created by use of the UDS\ strobe line from the CPU.
Address lines A1 through A13 are supplied by the address lines from the CPU.
Address lines A14 through A19 are supplied from an 8 bit port, bits 0 through 5.
42 FONIX 6500-CX Maintenance Manual