Specifications

Troubleshooting Guide for Mode 20

If you’re having trouble with Mode 20 operation, please study the following symptoms and

possible causes. The most common problems are listed below.

Symptom: The counter does not count.

Possible causes:

1. Field sensor and wiring – Verify that the encoder or other field device inputs actually turn on and

illuminate the status LEDs for X0 and X1. A standard incremental encoder will visibly, alternately

turn on the LEDs for X0 and X1 when rotating slowly (1 RPM). Or, the problem could be due to

a sinking-sourcing wiring problem, etc. Remember to check the signal ground connection. Also

verify that the pulse on-time, duty cycle, voltage level, and frequency are within the input

specifications.

2. Configuration – make sure all of the configuration parameters are correct. V7633 must be set to

20, and V7634 must be set to “0002” to enable the Phase A input, and V7635 must be set to

“0000” to enable the Phase B input.

3. Stuck in reset – check the input status of the reset input, X2. If X2 is on, the counter will not

count because it is being held in reset.

4. Ladder program – make sure you are using counter CT174 in your program. The top input is the

enable signal for the counter. It must be on before the counter will count. The middle input is the

dummy input and must be off for the counter to count. The bottom input is the counter reset, and

must be off during counting.

Symptom: The counter counts in the wrong direction

(up instead of down, and visa-versa).

Possible causes:

1. Channel A and B assignment – It’s possible that Channel A and B assignments of the encoder

wires is backwards from the desired rotation/counting orientation. Just swap the X0 and X1 inputs,

and the counting direction will be reversed.

Symptom: The counter counts up and down but will not reset.

Possible causes:

1. Check the LED status indicator for X2 to make sure it is active when you want a reset. Also verify

the configuration register V7636 for X2 is set to 7. Or, if you are using an internal reset, use the

status mode of DirectSOFT to monitor the reset input to the counter.

DL06 Micro PLC User Manual, 3rd Edition, Rev. C

E–37

Appendix E: High-speed Input and Pulse Output Features