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SSIO-24 USER GUIDE
2.4 MODE SELECTION BLOCK J3
The SSIO-24 uses a “power-up” circuit to ensure that output modules
will power up in the OFF state when the PC is turned on. This circuit
operates in either the Normal Mode or the Protected Mode, depending
on the setting of Mode Selection Block J3. The two operating modes are
discussed in the subsections that follow.
Normal Mode
J3 = NORM is the factory setting for the Mode Selection Block.
Powering up the PC resets the PIO-XX board, making all outputs of the
PIO-XX board go active HIGH. At the time a PIO-XX port is
programmed for output, all the lines of that port go inactive LOW. The
SSIO-24 monitors this transition from a logic HIGH level to a logic
LOW level at the
Þ
rst bit of each of the four (4) ports. As soon as the
SSIO-24 detects this transition at a port, it enables the OUTPUT
modules connected to that port. INPUT modules are always enabled
(SW1 setting = INPUT). The SSIO-24 must detect this HIGH to LOW
transition for the OUTPUT modules to become enabled.
Two anomalies exist with NORMAL mode because a reset line to the
SSIO-24 is unavailable. The anomalies are obvious only when using
OUTPUT modules.
After a PIO port is programmed for OUTPUT and the reset button of
the PC is pushed, ALL OUTPUT MODULES GO ACTIVE. This
activation is a potential problem for applications sensitive to such an
event. Either use a
warm reset
(by simultaneously pressing [Ctrl] + [Alt]
+ [Delete] on the PC) or a
cold reset
(turn off the PC, then restart it). A
third alternative is explained under
Protected Mode
, below.
When powering down the PC after the OUTPUT modules are enabled,
the OUTPUT modules become ACTIVE for several milliseconds before
being shut down by the absence of power. This event can be annoying
or potentially dangerous, based on the application. If this behavior is
unacceptable, refer to the
Protected Mode
, below.
