Name: Tandy 1000
Brand: Tandy
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Tandy 1000

Technical Reference Manual

Theory of Operation for the 300 Baud Modem

Modem Filter

The modem filter section consists of U3, parts of U9 and U6, and the modem

chip U1. U3

MC145440 filter chip.

contains two switched capacitor

filters and an operational amplifier. Depending on the mode of operation, one

filter processes the receive carrier, while the other filter processes the transmit

carrier. The operational amplifier

used

a duplexer. This device allows

transmit carrier to be amplified to the telephone interface transformer but not

to the receive side of the filter.

The

filter chip

Internal circuitry generates a false ground at

which

one-half the Vcc

power of the chip.

All

signals within the filter chip swing around this level. The

false ground

shown

a chassis ground symbol on the schematic, and the

true ground (0.0

VDC)

shown with a signal ground symbol

triangle).

U3 uses a 4.00 MHz

(Y1)

crystal to generate an internal clock. The frequency

also divided by 4 to generate a 1.00 MHz clock for the modem chip. This

clock reference

supplied

of U3.

Transmit Section

Serial data from the UART

supplied

of U8. The modem chip, U

frequency modulates the high and low state of the input

and generates

transmit carrier. Transmit carrier

supplied to the filter from

9 of U

digitally-generated sine wave

produced

this pin and

DC decoupled by

C2.

R1,

a 10K pot, adjusts the level of transmitter carrier to the filter chip

The signal

passed through summing amplifier U2 and applied to the

duplexer input.

passes most of the signal energy to

and on

the secon-

dary of

T1.

Receive Section

The receive carrier passes through

and

applied to pin 17. Transmit car-

rier

sensed

pin 15 of U3 and

subtracted from the smaller received car-

rier by the duplexer op-amp

U3. The duplexer's output

16,

and

the receive filter input

tied to t1is pin. Once the receive carrier

processed

by the receive filter, the output

decoupled away from false ground

by C13 and allowed to swing arouna system ground. R16, R21, and part of

U6 give about 10 dB of gain, and the received carrier is filtered by

R29

and

C23. C22 decouples the signal back to the false ground reference, and the

signal

applied

the limiter, U9. After being squared

the limiter, the modem

chip

detects any

FSK

activity and generates a serial output for the UART

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

Tandy 1000 Technical Reference Manual

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