Display Ram (U3, U9); Character Fonts (U2); System Clock (Us) - Wyse WY-120 Maintenance Manual

Table 4-2 Pin Assignments
for Custom IC U4, Continued
Display RAM (U3, U9)
Character Fonts (U2)
System Clock
(US)
Section 4
Description Pin Direction
AD 0-7 41-50, 52, 54 In/Out
54, 56, 58
DADD 0-14 2, 84-87, 89, Out
91-97, 99, 100
Video 0 64 Out
Video 1 61 Out
The custom IC generates the frame image and converts the data
bytes of the character, attribute, and font RAM into a four-level
pixel stream that goes to the CRT.
In normal operation the processor initializes the custom IC by
writing to each of the 12 screen attribute bytes (SABS) and the
row attribute byte (RAB). These bytes control all the video
display timing.
4-5
The display RAM consists of two 8Kx8 static RAMs. U3 stores
the display character data. U9 stores the corresponding attribute
byte for the character data in the hidden attribute mode. Both
store characters and relative attributes in nonhidden attribute
mode. The attribute RAM is backed up by a battery circuit that
maintains the contents of the RAM when power is lost. Maximum
access time for the RAM is 110 ns. The terminal's configuration
(soft setup) is stored in the attribute RAM.
Four 128-character fonts are stored in the 8Kx8 static RA'.\1.
Fonts are loaded into the RA'.\1 from the system PROM during
power-up and can be downloaded by an application program.
The system clock is derived from a 48-MHz Clapp oscillator built
around US and Yl (Figure 4-4). Of the 360 degrees of phase
shift needed for oscillation, the inverter supplies 180 degrees,
while capacitors CB and C21 supply 90 degrees each. L 1 and C8
form a resonant circuit to force the crystal to resonate at its third
overtone. RlS and R16 form the feedback path from input to
output. Capacitor C6 filters any AC signal from the bias network,
R15 and R16. Capacitor C7 removes DC bias from the circuits.
The output buffer is designed to provide a 48 percent to 52
percent square wave over the terminal's operating voltage and
temperature range. The resulting dot clock shifts the serial video
data from the custom IC U4 to the video amplifier. Dot clock is
used for all internal timing.