Page 28 - CNC Robotics

Chapter 2 / Electronics

motor. The L297 measures the voltage drop across these resistors
to control the PWM chopper circu it used to control the current
within the windings of the steppe r motor. The 2.2K and 10K resis-
tors connected to pin 15 (Vref) on the L297 are used to set up a
voltage divider. The resulting voltage applied to the Vref pin is
used as a set point against the measured voltage coming from the
field windings. When the set po int is reached, the power drive
stage within the L298 driving that winding is turned off, allowing
the FR304 diodes to discharge the field winding. The field winding
sta ys off until the internal oscillator within the L2 97 times out and
turns the field winding back on . The 22K resistor and the 3.3nF
capacitor connected to pin 16 on the L297 set up the timer's chop-
per rate. C2, C3, and C4 filter the power supply for the elect ronics
and motors. jP I pro vides power and ground for the circuit and the
motors. jP2 is jumped to include the circuit connected to pin I with
pin 16 on the L297 on only one board of the three used; the other
two have pin 16 on the L29 7 jumped to ground. JP3 pin I accepts
step signals; pin 2 accepts direction signals; pin 3 is used to con-
nect all driver boards that need to be synced and pin 4 is ground.
jP4 is set to bring pin 19 on the L297 high at pin 1 to provide full
step motor drive or low at pin 3 for half step. JPS is set to bring pin
li on the L297 high at pin I to use phase driving or low at pin 3
to inhibit drive.
Don 't worry too much about und erstanding how this circuit wor ks;
as long as you follow the directions closely, the board you build
will function properly.

The Interface Board

This board is really only a gateway that allows the computer to
send and receive signals to the drivers and limit switches. There
isn 't much to it aside from a connector for the straight-through
parallel port cable and so me co nnections for the wires coming
from each of the driver boards and limit switches. If you refer to
the schematic (Figure 2.7) you will see wh y the parts list is short.

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