Ramsey Electronics FZ-146 Manual de instrucciones descargar pdf (Pagina 20)

P-IBM • 20

P-IBM MODEM CIRCUIT DESCRIPTION:

The Texas Instruments TCM3105 FSK Modem IC (U2) is the heart and soul of

this proven modem circuit. The functions of the IC pins are designated on the P-

IBM schematic diagram. In fact, the schematic itself makes most circuit

functions fairly obvious.

U1 is a hex inverter, meaning that it contains six inverters in a single DIP IC

package.

The power input circuitry (VR1,C11,D2,D4,D6,D7 etc.) looks for ANY voltage at

either the RS232C computer connection or pin 5 of the transceiver DIN jack and

provides the regulated +5 VDC required by the modem.

Transmit data from the serial port is converted from RS232C levels (+/-12V) to

TTL levels (+5V and 0V) by inverter U1:a. U2 converts the data to Bell 202

tones (1200 and 2200 Hz), with these frequencies governed by the precision of

Y1. The modulation level of +/- 4 KHz is set by trimmer R9.

Packet AFSK audio tones from the receiver through pin 4 of the DIN connector

are converted by U2 to digital data which is fed to the computer serial port by

U1:C. The Data High and Data Low LED's (D3,D5) are driven by U1:B and

serve the Receive (RXD) line only.

The PTT signal from the computer is converted to a TTL level by U1:D. A "Time

Out" circuit, set for about 40 seconds, is formed by C2,R3,D1 and U1:E. Its

purpose is to unkey the transmitter PTT after that amount of time if the

computer 'locks' up. Q1 is a transceiver switch which will handle virtually any

modern transceiver PTT switching requirement.

Circuit points P2,P3,P11,P15 and P18 are brought out on the PC board to

permit use of the IBM-compatible parallel printer port as may be required by

some packet software programs. The serial TXD and PTT lines must be

disconnected by omitting JMP1 and JMP2. See the detailed discussion of this

application on page 15.

REVIEW:

1. Voltage may be supplied by either computer or transceiver.

2. The modem is designed for Bell 202 tones (1200,2200 Hz) standard for

VHF packet operation.

3. The Mike Level trimmer (R9) should be set for +/-4 KHz FM deviation.

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Ramsey Electronics FZ-146 Manual de instrucciones Pagina 20