Omron NT21 Reference Manual page 434

Troubleshooting
PT Symptoms
The display is too faint.
The digits in numeral
display come out as *.
420
Cause
The contrast is too high.
The number of digits of a numeric
value in a numeral memory table
entry exceeds the set number.
Remedy
For the NT31/NT31C or NT631/NT631C, use the
system menu to reduce the contrast.
(Refer to section 6 of the Setup Manual.)
For the NT21, use the switches on the back of the
PT to increase the contrast.
Confirm the values of the PC words allocated to the
numeral memory table entry.
Allocate one PC word to each numeral memory
table entry.
Section 6-1