Chapter 4
Connecting Signals
©
National Instruments Corporation
4-7
Types of Signal Sources
When making signal connections, you must first determine whether the
signal sources are floating or ground-referenced. The following sections
describe these two types of signals.
Floating Signal Sources
A floating signal source is not connected in any way to the building ground
system but, rather, has an isolated ground-reference point. Some examples
of floating signal sources are outputs of transformers, thermocouples,
battery-powered devices, optical isolator outputs, and isolation amplifiers.
An instrument or device that has an isolated output is a floating signal
PFI3/GPCTR1_SOURCE
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
PFI4/GPCTR1_GATE
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
GPCTR1_OUT
DO
—
—
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
PFI5/UPDATE*
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
PFI6/WFTRIG
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
PFI7/STARTSCAN
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
PFI8/GPCTR0_SOURCE
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
PFI9/GPCTR0_GATE
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
GPCTR0_OUT
DIO
—
V
CC
+0.5
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
FREQ_OUT
DO
—
—
3.5 at
(V
CC
–0.4)
5 at 0.4
1.5
50 k
Ω
pu
pu = pull up; pd = pull down; DO = Digital Output
The tolerance on the 50 k
Ω
pull-up and pull-down resistors is very large. Actual value may range between 17 and 100 k
Ω.
Table 4-5.
Digital I/O Signal Summary (Continued)
Signal Name
Signal
Type and
Direction
Impedance
Input/
Output
Protection
(Volts)
On/Off
Source
(mA at V)
Sink
(mA at V)
Rise
Time
(ns)
Bias