Serial Data Chain; Daculator Circuit - Tektronix 2212 Service Manual

Serial Data Chain
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The 2212 is fully microprocessor controlled. To prevent digital noise,
glitches and other polluted signals that can disturb the sensitive circuitry of
the scope, the microprocessor controls the analog section of the scope by
way of a serial data chain.
There is no system bus activity or other microprocessor activity on the
mainboard of the 2212 oscilloscope. Six bytes are required to set up the
analog part of the scope.
Byte 1.
Byte 2.
Byte 3.
Byte 4.
Byte 5.
Byte 6.
U800 contains the control byte for the trigger source- coupling-
and slope selection.
U801 contains the controlbyte for the trigger mode that sets the
sweep logic.
U802 contains the controlbyte for the sweep speed.
U803 contains the controlbyte for the CH 1 sensitivity.
U804 contains the controlbyte for the CH 2 sensitivity.
U805 contains the control byte for the vertical mode selection.
The six single shift registers are cascaded to one 48 bit long shift register,
called the data chain. When new data must be shifted into the chain, the
microprocessor places bit by bit on the serial data line (SER_DAT A) and
shifts it into the chain with the serial clock signal (SER_CLK). The shift
registers are not transparent, so a bit that is moving through the chain does
not affect the outputs. When the 48 bits word is loaded, a strobe pulse
(CHAIN_LD) clocks all bits in parallel to their outputs. If one bit of the
chain has to be changed, a complete new 48 bits wide word must be loaded
into the chain.
Daculator Circuit
The 2212 scope has a 16 channel DAC, called the daculator. The outputs of
this DAC are used to control and calibrate the scope. The individual out-
puts are discussed in other parts of the circuit description. The daculator is
loaded the same way as the serial data chain with a 16 bits wide word.
3 - 18
12 Bits for the DAC and 4 bits to address the output channel.
The daculator uses the same serial data line (SER_DA TA) and the same
clock signal (SER_CLK) and has its own strobe pulse, (DACU_LD). So all
the code information is loaded into both shift registers, the serial data chain
and the daculator. The CHAIN_LD and the DACU_LD pulse however
determine which circuit actually accepts the data.
The daculator is completed with some peripheral circuitry to improve the
performance. An external reference is used to have a better accuracy and
lower drift. An RC filter and a buffer circuit at each output are provided to
reduce digital noise and increase the output drive.
Theory of Operation