National Instruments 321645c-01 用户手册

Chapter 2

Function Reference — GPCTR_Change_Parameter

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 National Instruments Corporation

2-207

NI-DAQ FRM for PC Compatibles

Software Control
This function lets you customize the counter for your application. You can use this function 
after the 

GPCTR_Set_Application

 function, and before 

GPCTR_Control

 function with 

action = 

ND_PREPARE

 or action = 

ND_PROGRAM

. You can call this function as many times as 

you need to.

The software up or down control is available by default; if you do not use the 

GPCTR_Change_Parameter

 function with paramID set to 

ND_UP_DOWN

, the counter is 

configured for the software up or down control and starts counting up. To make the counter 
use the software up or down control and start counting down, use the 

GPCTR_Change_Parameter

 function with the paramID set to 

ND_UP_DOWN

 and the 

paramValue set to 

ND_COUNT_DOWN

. To change the counting direction during counting, use 

the 

GPCTR_Control

 function with the action set to 

ND_COUNT_UP

 or 

ND_COUNT_DOWN

.

Hardware Control
To use hardware to control the counting direction, use I/O connector lines as shown in 
Table 2-29; the counter will count down when the I/O line is in the low state and up when it 
is in the high state. Use the 

GPCTR_Change_Parameter

 function with the paramID set to 

ND_UP_DOWN

 and the paramValue set to 

ND_HARDWARE

 to take advantage of this counter 

feature.

ND_BUFFER_MODE

 (6602 and 455X devices only)

Corresponding legal values for paramValue are shown below:

•

ND_SINGLE

 

for single buffer operations.

•

ND_DOUBLE

 for continuous buffer operation.

Table 2-29.  Default Up/Down Selection

ND_COUNTER_0

Digital I/O Line 6

ND_PFI_37

ND_PFI_29

ND_COUNTER_1

Digital I/O Line 7

ND_PFI_33

ND_PFI_25

ND_COUNTER_2

N/A

ND_PFI_29

N/A

ND_COUNTER_3

N/A

ND_PFI_25

N/A

ND_COUNTER_4

N/A

ND_PFI_21

N/A

ND_COUNTER_5

N/A

ND_PFI_17

N/A

ND_COUNTER_6

N/A

ND_PFI_13

N/A

ND_COUNTER_7

N/A

ND_PFI_9

N/A