Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Description of Schematics 10-7
SRS Residual Gas Analyzer
In addition to the I/O port on the microcontroller, there are three 8-bit digital shift
registers which are loaded via the SPI, then strobed by a LD bit to transfer data to the
parts’’ output registers. The three digital output ports are assigned as follows:
The LED port
Q0 Set high to light an LED to indicate that the +24V power supply is ““okay””.
Q1 Set high to light an LED to indicate that the filament is ““on””.
Q2 Set high to light an LED to indicate that the unit is degassing.
Q3 Set high to light an LED to indicate that the CDEM high voltage is ““on””.
Q4 Strobe high to set I/V converter to LOG mode
Q5 Set high to light an LED to indicate that there has been an error.
Q6 Set high to light an LED to indicate that the pressure is too high.
Q7 Set high to light an LED to indicate that the filament is burnt-out.
The MPX port
Q0 CAL_0 LSB of current detector calibration attenuator multiplexer.
Q1 CAL_1 Middle bit of current detector calibration attenuator multiplexer.
Q2 CAL_2 MSB of current detector calibration attenuator multiplexer.
Q3 MPX_0 LSB of 16-bit A/D converter's input multiplexer.
Q4 MPX_1 Middle bit of 16-bit A/D converter's input multiplexer.
Q5 MPX_2 MSB of 16-bit A/D converter's input multiplexer.
Q6 EMIT_CTL Filament heater duty cycle control: 0=direct, 1=regulate
Q7 GRID_SEL Low for low grid potential, high for high grid potential.
The MISC port
Q0 RF_ON Low to turn-off RF, high to turn on.
Q1 IONS_OFF Pulse high for 50 ms to reset relay and ground FC
Q2 IONS_ON Pulse high to set latching relay so FC signal enters I/V.
Q3 MULT_OFF Pulse high to ground e-multiplier anode.
Q4 MULT_ON Pulse high to connect e-multiplier to I/V.
Q5 LINEAR Pulse high for linear I/V converter (offset calibration).
Q6 R/-C Set high to read 16-bit A/D, low to initiate conversion.
Q6 CS_VETO Set high to prevent (or release) -CS to 16-bit A/D converter.
Clocks
The master clock of the system is an 11.0592 MHz crystal oscillator. This frequency was
selected for the following reasons:
1. It is close to the maximum operating frequency for the 68HC11 (12 MHz).
2. When divided by four it provides a usable RF frequency (2.7648 MHz).
3. When divided by 128 it provides a clock for switching power supplies (86.4 kHz).
4. When divided by 256 it provides a clock for the high voltage DC-DC converter.
5. When divided by 384 it provides an exact PC baud rate (28.8 kbaud).
6. When divided by 2 14 it provides a A/D convert rate (675 Hz).