CY545 Stepper System Controller
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To read an entire parameter, the digit read process is repeated three, five, or eight times,
depending on the size of the parameter. The CY545 starts by reading the most significant digit,
then the next, until it finally reads the least significant digit. This is the same order that digits are
read from a command with numeric arguments.
The various CY545 commands have been split into three groups for parameter addressing
purposes. This organization minimizes the address decoding logic needed to access the
external switches.
The applications schematic includes an example interface to three different parameters, the
Number parameter with 5 digits, and the Rate and First rate parameters, each with 3 digits.
Each group of 8 parameters is selected by a 74LS138 decoder, which generates an enable
signal for the selected command parameter. Each digit of the enabled parameter is uniquely
enabled by a signal from a 74LS156 decoder. Digits are enabled in address sequence, with the
least significant digit selected by the Y0 output selection from the 74LS156.
For 5 and 8 digit parameters, one 74LS156 must be used to address the parameter. It can
generate up to 8 enable signals, but 3 will not be used in the 5 digit cases. The Delay parameter
would be an example of this.
For 3 digit parameters, one 74LS156 can be used to address two different parameters. Each
half of the 74LS156 can select 3 digits, with one line not used. The Rate and First rate
parameters illustrate this example.
To allow every parameter to be specified by a thumbwheel switch selection would require three
74LS138s, plus one or one half 74LS156 per parameter. It also would take one thumbwheel
switch per parameter digit. This could be a significant amount of hardware if all parameters are
externally controlled. However, we expect that in real applications, only a few parameters will
need this feature, and hardware complexity will not be that bad.
The thumbwheel switches used for each digit should be BCD encoded, and must have isolation
diodes between the digit lines. Some switches have these diodes built in, but most do not. The
diodes have been shown on the example schematic. Also, with the parts shown, the switches
must have a complemented common signal to generate the proper values at the CY545 data
bus. By using an inverting buffer to the CY545 data bus, a non-complemented common signal
switch could also be used. Pull-up resistors provide a high signal for those digit lines not
connected to the common for a particular digit value.
The following section gives the address values for each parameter the CY545 can read from
external switches. The lowest number of the range corresponds to the least significant digit of
the parameter, and is read last by the CY545. The highest number is the most significant digit
address, and is read first.
© 2002 Cybernetic Micro Systems
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Chapter 13 - Thumbwheel Switch Support
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