TECHNICAL MANUAL FOR COMPACT VHF DSC RM2042

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RM2042Output pin PB4 is used to control a remote alarm circuit.Output pin PB5 is used to control the relay on the interface module.Output pin PB6 is used to control output from the syncronus transmitter U26, and the modem IC on thereceiver module. A logic low level starts the transmission, and a logical high level ends the transmission.Output pin PB7 is used to control the generation of alarm tones by U21.1. A logical low disables, and alogical high enables the output from U21.1.Input pin PC0 is used to determine the status of the key input from the PTT.Input pin PC1 is used to determine the status of the carrier detect.Input pin PC2 is the serial bus interrupt request from the connected VHF.Input pin PC3 is a signal from the power fail circuit, telling that VBAT is dropping below 10V.4.9152 MHz OSCILLATOR WITH DIVIDERThe 4.9152 MHz oscillator is build as a gate oscillator around U16, with the crystal X2 to control theoscillation frequency. The oscillator is used as a reference clock for the divider that generates baud ratesfor syncronus and asyncronus communication.PRINTER STROBE GENERATORWhen the STRB(P27) signal from the interface module goes low, U14.2 generates, after delay of 2 Us, a4 U s logic low strobe pulse to the printer (STROBE (LPT)).MODEM COMMUNICATION INTERFACEThe modem communication interface U26, is a syncronus 1200 baud receiver/transmitter. The receiveris controlled by a an external SYNC signal, and a syncronus sampling clock. The SYNC signal is theMODULATION DETECT signal from the channel-70 receiver. When this signal goes high, and the firsthigh going edge on RXDATA is detected, the syncronus receiver clock is started and RXDATA is sampled.Each bit is sampled 8 times (RXCLX is 9600 Hz), and whenever one bit is received, RXRDY goes high,and the microprocessor is interrupted. Data is then fetched by the microprocessor, for further processing.U26 converts data it receives via the parallel data bus to serial data. The data is output on TXDATA, if/when the CTS input pin is low. Whenever the U26 is ready to send another byte, TXRDY goes high andinterrupts the microprocessor. If the microprocessor has more data to send, another byte is written to U26.Transmission is disabled by setting the CTS input high.VOLTAGE CONVERTERThe voltage converter U32 is used to generate -5V from the +5V supply. The -5V is used to control theview angle of the lcd dot-matrix display.KEYBOARD LIGHT ON/OFFThe two transistors Q2 and Q3 are used to control the keyboard light. When PB0 on U22 is high Q2 andQ3 is on, and VBAT is supplied to the LED’s on the keyboard module.9341PAGE 5-9