Fluke 123B/S Service Manual - Page 57

3 Circuit Descriptions 3.3 Detailed Circuit Descriptions Keyboard Control, ON/OFF Control The keys are arranged in a 6 rows x 6 columns matrix. If a key is pressed, the D-ASIC drives the rows, and senses the columns. The ON/OFF key is not included in the matrix. This key toggles a flip-flop in the D-ASIC via the ONKEY line (D-ASIC pin F4). As the D-ASIC is permanently powered, the flip-flop can signal the test tool on/off status. SDA-SCL Serial Bus The unidirectional SDA-SCL serial bus (pin A2, A3) is used to send control data to the C-ASIC's (e.g. change attenuation factor), and the T-ASIC (e.g. select other trigger source). The SDA line transmits the data bursts, the SCL line transmits the synchronization clock (1.25 MHz). TXD, RXD Serial Interface (Optical Port) The optical interface output is directly connected to the TXD line (pin L1). The optical input line is buffered by the P-ASIC on the power part. The buffered line is supplied to the RXD input (pin L2). The serial data communication (RS232) is controlled by the DASIC. Slow ADC Control, SADC Bus The SELMUX0-2 (pins N1, N3, N4) and SLOWADC (pin A4) lines are used for measurements of various analog signals, as described in Section 3.3.1. "SLOW ADC". BATIDENT The BATTIDENT/BATIDGAR line (pin B5) is connected to R508 on the Power part, and to a resistor in the battery pack (0 Ω for Ni-Cd, 825 Ω for Ni-MH). If the battery is removed, this is signaled to the D-ASIC (BATTIDENT line goes high). MAINVAL, FREQPS The MAINVAL signal (pin M2) is supplied by the P-ASIC, and indicates the presence of the power adapter voltage (high = present). The FREQPS signal (pin M3) is also supplied by the P-ASIC. It is the same signal that controls the Fly Back Converter control voltage FLYGATE. The D-ASIC measures the frequency in order to detect if the Fly Back Converter is running within specified frequency limits. D-ASIC Clocks A 32 kHz oscillator runs if the 3V3GAR supply voltage is present, so if any power source is present (crystal B401). The clock activates the Power On/Off control circuit and the instrument's Real Time Clock (time and date). A 50 MHz oscillator runs if the test tool is ON, and/or if the power adapter voltage is present (B403). A 3.6864 MHz UART oscillator for the serial RS232 communication runs if the 50 MHz oscillator runs (B402). Buzzer The Buzzer is directly driven by a 4 kHz square wave from the D-ASIC (pin T4) via FET V522. If the test tool is on, the -30VD supply from the Fly Back converter is present, and the buzzer sounds loudly. If the -30VD is not present, the buzzer sounds weak, e.g. when the Mask Active mode is entered. 3-29