PIO-12 Data Acquisition Board Design
Introduction
CAUTION
This PIO-12 is a slightly different design than earlier production PIO-12s. This change occurred in all
PIO-12s manufactured after November 1996, as a result of a change by the manufacturer of the 8255 to
the 82C55 chip with C-MOS technology . Newer production boards incorporating this change can be easily identified by the 82C55 part number on the main IC on the board. (The older design contains an
8255A or a 71055C part as the main IC.) The primary effect of this change is that in some applications
some devices (such as relays) connected to the outputs may be turned “ON” when the board is first powered up. After the board is initialized and the ports are configured as inputs or outputs and set to the
desired state, the board will operate in the same manner as previous version production boards. If an
“ON” condition prior to initialization presents no problems or hazards or if the system is designed such
that no such condition occurs, the current PIO-12 board can be used in applications designed to use the
original PIO-12. Refer to the following paragraphs for technical details regarding the potential “ON”
condition.
If the potential “ON” condition during power-up cannot be tolerated, we recommend the use of the PIO-24, which does not
exhibit this circuit behavior. The PIO-24 has the same pinout as the PIO-12. Refer to Figure 1.
Note
The PIO-24 does not support modes 1 and 2 of the 8255, which is required in some special applications.
19
37
18
36
Connector Connector
PIO-12 PIO-24
17
35
16
34
15
33
14
32
13
31
12
30
11
29
10
28
9
27
8
26
7
25
6
24
5
23
4
22
3
21
2
20
1
Connector Pin Assignments Connector Pin Assignments
19
37
18
36
17
35
16
34
15
33
14
32
13
31
12
30
11
29
10
28
9
27
8
26
7
25
6
24
5
23
4
22
3
21
2
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1
Figure 1
Models PIO-12 and PIO-24
Technical details
The PIO-12 (original design) I/O ports are tri-state designs as shown in Figure 2. At power up, all I/Os are set to a high impedance mode as inputs. Relays, transistors, etc. connected to these pins will remain in an “OFF” state during power-up and board
initialization. The outputs will then be controlled by their software program. Whether “HI” or “LO” is considered, “ON” depends on the circuit connected to the output.
The PIO-12 (newer production) I/O ports, as shown in Figure 3, incorporate an internal pull–up to +5V DC on each pin. This
circuit can source as much as 400µA whenever 5V DC is applied to the board. As shown in Figure 4, this can ener gize low po wer
PA-612 Rev. A / 9-98
relays, some solid state relays, and transistors (such as input transistors on an ERB-24 relay board), turning such devices to an
“ON” state during power-up. Once the board is initialized, any I/O port set to an output state will go to appropriate “HI” or “ LO”
state and act in the same manner as an original design PIO-12. Note, as show in Figure 5, output devices connected to the PIO-12
in a “LO = ON” configuration will not experience problems during the PIO-12 power-up sequence. Also note that the SSIO-24
board contains circuits that permit the use of any PIO-12 version (original design or new) without e xperiencing problems during
the PIO-12 power-up sequence.
+5V
All port
A,B,C pins
400µA
All port
A,B,C pins
Figure 2
Original PIO-12 I/O stage (with 8255)
400µA 400µA
Relay
Solid State Relay ERB-24
Figure 4
Examples of output devices and configurations that can
be turned “ON” by a PIO-12 during board initialization
+ 5V + 5V
From
PIO-12
Figure 3
New production PIO-12 I/O stage (with 82C55)
+ 24V
400µA
From
PIO-12
Figure 5
Examples of output devices and configurations that will not
be turned “ON” by a PIO-12 during board initialization
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