ALLEN BRADLEY 1747-L551 PLC Processor

8–6
Starting Up Your Control System
Input Troubleshooting Steps
1. Make sure the processor is in the Continuous Scan Test mode.
2. If associated bit status and LED status do not match the input
device status, check status file S:11 and S:12 I/O slot enables.
Bits S:11/0 through S:11/15 and S:12/0 through S:12/14 should
all be one, enabling all I/O slots for the modular system.
3. Verify proper control power to the input device.
4. Remove the input device power and make sure circuit
terminations are properly wired and tightened.
5. Re-energize the input device power and check for proper control
voltage between the input terminal and signal common terminal.
6. If proper input control voltage does not exist, first check
minimum signal common by verifying voltage between input
device power source and the input common terminal.
7. If proper input voltage does exist, first check the minimum input
current specification on the input module and then measure the
current in the input circuit. Replace the input module if
necessary.
8. If the input modules check out “OK” and proper voltage is
measured between input device source and input module common
terminal, test the input device and replace if necessary.
For more information on input troubleshooting see page 10–26.
Publication 1747-6.2