Tandy 1000 Technical Reference Manual

Tandy 1000
noise generator
Technical Reference Manual
TYPE SN76494. SN76496
PROGRAMMABLE TONE/NOISE GENERATOR
The noise generator consists of a noise source and an attenuator. The noise source is a shift register with an
exclusive OR-feedback network. The feedback network has provisions to protect the shih register from being locked
in the zero state.
TABLE 2 . NOISE FEEDBACK CONTROL
FEEDBACK
o
1
CONFIGURATION
"Periodic" noi ..
"White" "aiM
Whenever the noise control register is changed. the shift register is cleared. The shift register will shift at one of four
rates as determined by the two NF bits. The fixed shift rates are derived from the input clock.
TABLE 3 - NOISE GENERATOR FREOUENCY CONTROL
BITS
SHIFT RATE
NFO
NFl
0
0
N/64
0
1
N/128
1
0
N/256
1
1
Tone generator #3 output
The output of the noise source is connected to a programmable attenuator as shown in Figure 4.
output buffer/amplifier
The output buffer is a conventional operational amplifier summing circuit. It sumS the three tone generator outputs,
the noise generator output, and any audio input through pin 9. The output buffer will generate up to 10 mAo
To prevent oscillations in the output buffer. the output (pin 7)should be decoupled. This is done by putting 10 ohms in
series with 0.1 pF from pin 7 to ground (see figure 3).
data transfer
The microprocessor selects the SN76494 by taking CE low (low voltage). Unless CE is low. no data transfer can occur.
When CE is low. the WE signal strobes the contents of the data bus to the appropriate control register. The data bus
contents must be valid at this time.
The SN76494 requires approximately four clock cycles to load the data into the control register. The SN76496 requires
approximately 32 clock cycles. The open-collector READY output is used to synchronize the microprocessor to this
transfer and is pulled to the false state (low) immediately following the falling edge of CE.lt is required to go to the true
state (high) when the data transfer is completed. The data transfer timing is shown below.
:TPHl~
"lADY---~.1.'\I _~ ~"'OM
: ~....... ~-----YOL
, I
~ r-- """'(1 r--- ftI:
FIGURE 2 - DATA TRANSFER TIMING

Tandy 1000 Specifications

General

Processor	Intel 8088
Processor Speed	4.77 MHz
RAM	128 KB (expandable to 640 KB)
Operating System	MS-DOS 2.11
Manufacturer	Tandy Corporation
Release Year	1984
Storage	Single or double 5.25" floppy disk drive (360 KB), optional hard drive
Sound	Tandy 3-voice sound
Ports	Parallel, Serial
Graphics	CGA (Color Graphics Adapter)
Graphics Modes	320x200, 640x200