GE 90-30 PLC Series Installation And Hardware Manual page 339

13
Reference Tables
There are two types of reference tables, standard and mixed. These tables show groups of memory
addresses and their status. For discrete addresses, the status of these will be shown as either logic 1
or logic 0. For analog and register addresses, values will be displayed. Standard tables display
only one type of memory address, such as all of the %I bits. Mixed reference tables are created by
the user, who selects what addresses to display in the tables. These mixed tables can contain
discrete, analog, and register references all in one table. This makes them useful for gathering
numerous related addresses on one screen where they can all be viewed or monitored at the same
time. This saves time compared with searching or scrolling through the ladder logic screens to find
these addresses.
Override feature
This feature must be used with caution to ensure the safety of personnel and equipment.
Normally, the machine should not be cycling, and all conditions should be such that the
output device can be turned on without any harm being done. This method can be used to
check an output circuit from the ladder screen all the way to the device being controlled. For
example, when overriding and toggling a %Q output to an ON state, the relay, solenoid, or other
device being controlled should turn on or pick up. If it does not, the status light on the output
module could be checked, then voltage checks could be made at the module terminal board, the
system terminal strip, the machinery terminal strip, the solenoid or relay connections, etc. until the
source of the fault is found.
Sequential Event Recorder (SER), DOIO functional instruction
These can be set up to capture the status of specified discrete addresses upon receiving a trigger
signal. They may be used to monitor and capture data about certain portions of the program, even
when unattended. They can be useful for locating the cause of an intermittent problem. For
example, a contact in a string of contacts that maintain power to a coil may, from time to time,
momentarily open and interrupt normal operation. However, when maintenance personnel attempt
to locate the problem, all of these contacts may test OK. By using the SER or DOIO instruction,
the status of all of these contacts can be captured within milliseconds of the time the fault occurs,
and the contact that opened will show a status of logic 0 at the moment of capture.
13-4 Series 90-30 PLC Installation and Hardware Manual – August 2002 GFK-0356Q