SMAC
LAC-25 Technical Reference Manual
8
1.2 Digital I/O Interface
The LAC-25 includes 4 channels of general purpose digital input and 4 channels of
general purpose digital output. Additionally, there are four channels of dedicated digital input for
each of the two axis'. All of these I/O are protected through the use of optoisolators.
1.2.1 Dedicated Digital Inputs
Figure 1 illustrates one of the LAC-25Õs dedicated digital inputs. These inputs are Limit+,
Limit-, Home and Fault for both axis of the LAC-25. The power to activate these inputs is provided
by the LAC-25 so it is only necessary to complete the circuit to activate the input.
The Limit inputs are intended for signaling the LAC-25 that an axis has reached itÕs end of
travel. When such an event occurs, the LAC-25 can ignore the event or stop the servo in some
controlled fashion. The Home input is for detecting some sort of "home position" sensor. This can
be used with the encoder index input to implement a very accurate homing method. A typical use
for the Fault input is for an external device to signal a fault condition such as over-temperature.
Note:
The external Fault input is tied to the internal over-temperature signal from the onboard
drivers. When a fault condition occurs, that is either the internal over-temperature signal or
external Fault signal go active, the 16-bit internal variable FCNT (see Internal Variables)
begins to increment at 1 rate of once per millisecond. If the fault condition clears then the
FCNT variable is also cleared. If the fault condition remains present long enough for the
FCNT variable to count up to the value assigned to the FCMP variable, then the over Fault
bit in the status word will be set and the servo will be disabled (assuming the Fault interrupt
has not been enabled). The default value for FCMP is 10000 which will give a 10 second
delay before causing the Fault bit to be set.
Figure 1. LAC-25 Dedicated Input.
1.2.2 General Purpose Inputs
Figure 2 illustrates one of the LAC-25Õs general purpose inputs. These inputs are
galvanically isolated from the LAC-25. Current of the proper polarity must be supplied to the circuit
to activate the input.
