I use 95% thru-hole parts still, but I changed a dozen high-voltage capacitors to
1206 surface mount and I use a 16 pin SOIC chip for the analog multiplying DACs. I
will solder these parts onto the kit boards to save customers the trouble.
The power supply has been changed from a 110VAC powered design to a 12VDC powered
design. The analog voltage has been reduced to +/-5V from +/-12V. Voltage regulators
are provided for all low-voltage supplies. A sense bit allows the CPU to detect the
loss of 12V input power, and a control bit allows it to turn the display power on and
A pair of 1 Farad supercapacitors provide a minute or so of backup power to the CPU
I stole the circuit from the Tektronix 221 oscilloscope's Y output amplifier. It works
to 5 MHz (using faster transistors) and should have plenty of oomph for the circle
work I do. The main thing is that it be able to go to a new XY center position in a
microsecond or two.
The MC68HC908GP32 CPU provides 32K of ROM, 512 bytes RAM, lots of I/O ports, SPI,
serial, and versatile timers.
The programming port is shared with the RS-232 serial drivers. The programming mode
is entered via a DPDT slide switch. The MON08 operating mode is permanently selected.
The display supply control bit uses a pullup to turn off the display while
programming. The MAX232 generates the 10V Vtst for IRQ (used to enter MON08 mode)
as well as the RS-232 line driving voltage.
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Last updated March 13, 2008