Chapter 2
Hardware Overview
©
National Instruments Corporation
2-3
Table 2-1 summarizes the TTL inputs and outputs available on the
NI 8255R.
Isolated Inputs and Outputs
The isolated inputs and outputs on the NI 8255R have a separate ground
reference from the main NI 8255R supply, providing an easy means to
prevent ground loops that can introduce noise into a system. You can apply
signals up to 30 V to the isolated inputs. The voltage swing of the isolated
outputs is determined by the voltage you supply on the V
iso
pins of the
device.
Note
V
iso
can be supplied directly to the 44-pin D-SUB when using custom cabling.
Alternatively, V
iso
can be supplied directly to the 37-pin terminal block and to the NI Vision
I/O Terminal Block and Prototyping Accessory with the 44-pin to 37-pin NI cable.
Note
The isolated outputs have current-limiting protection circuitry. If this circuitry is
tripped, you can re-enable the outputs by removing the fault and restarting your computer.
Table 2-1.
NI 8255R TTL Inputs and Outputs
Primary
Function
Input or
Output
Number
Available
Signal Names
44-Pin D-SUB
on NI 8255R
Pin Number
37-Pin
Terminal
Block
Number
Trigger
Input
2
TTL Input 0,
General Purpose
*
TTL Input 1,
General Purpose
*
1
16
1
2
Timed
Pulse
Output
6
TTL Output 1, Pulse 1
TTL Output 2, Pulse 2
TTL Output 3, Pulse 3
TTL Output 4, Pulse 4
TRIG 1, Pulse 5
TRIG 2, Pulse 6
4
6
7
18
24
9
5
7
8
20
—
—
Watchdog
Output
1
TTL Output 0
3
4
General
Purpose
Output
3
TTL Output 5
TTL Output 6
TTL Output 7
19
21
22
21
23
24
* TTL Input 0 and TTL Input 1 can also function as trigger change detectors.
