Fluke ScopeMeter 123 Service Manual page 56

123
Service Manual
SDA-SCL Serial Bus
The unidirectional SDA-SCL serial bus (pin 56, 57) 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).
Probe Detection
Via the probe detection inputs PROBE-A and PROBE-B (pin 54, 55), the D-ASIC
detects if the Input A and B probes have been connected/disconnected. The SUPPRDET
signal (pin 99) can suppress the probe detection. If this signal is low, The PROBE-A and
PROBE-B lines are permanently low (via R471, R472), regardless of a probe is
connected or not connected. This function is not supported by the Fluke 123 software.
See also Section 3.3.2 "Probe detection".
TXD, RXD Serial Interface (Optical Port)
The optical interface output is directly connected to the TXD line (pin 86). The optical
input line is buffered by the P-ASIC on the power part. The buffered line is supplied to
the RXD input (pin 87). The serial data communication (RS232) is controlled by the D-
ASIC.
Slow ADC Control, SADC Bus
The SELMUX0-2 (pins 96-98) and SLOWADC (pin 100) lines are used for
measurements of various analog signals, as described in Section 3.3.1. "SLOW ADC".
BATIDENT
The BATTIDENT line (pin 90) is connected to R508 on the Power part, and to a resistor
in the battery pack. If the battery is removed, this is signaled to the D-ASIC
(BATTIDENT line goes high).
MAINVAL, FREQPS
The MAINVAL signal (pin91) is supplied by the P-ASIC, and indicates the presence of
the power adapter voltage (high = present).
The FREQPS signal (pin 93) 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 25 MHz crystal (B403) controls the D-ASIC system clock. For the real time clock,
counting the time and date, an additional 32.768 kHz crystal (B401) is provided. When
the test tool is turned on, a 16MHz microprocessor clock (derived from B402) becomes
active.
Buzzer
The Buzzer is directly driven by a 4 kHz square wave from the D-ASIC (pin 101) 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.
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