Siemens UM344-2 user manual Symptom Erratic Output, Diagnosing a Defective Digital LCD Meter

UM344-2CALIBRATION AND MAINTENANCE

	TABLE 6.9 Symptom - Erratic Output
POSSIBLE CAUSE		CORRECTIVE ACTION
Loop Wiring		Check for 12 Vdc minimum across SIGNAL +/- terminals in transmitter terminal
		compartment.
		Check power supply output voltage: 17 Vdc minimum; 42 Vdc maximum.
		Check for loose loop wiring at power supply terminals, Supply Barriers (if
		used), junction boxes, and transmitter terminal compartment.
		Check for loose leads at Current Sense Resistor.
		Check for accumulation of moisture in transmitter terminal compartment.
		Check for multiple grounds on loop cable shield.
		Check for the proper type of loop cable and for electrical interference between
		the loop cable and any adjacent cables in a cable tray or conduit.
		Check signal loop is grounded, not floating. Ground signal loop at one point
		only, preferably at the power supply negative terminals.
Loop power supply excessively		Confirm defective power supply by substituting spare supply for suspect
noisy		supply.
		Select a higher damping value (software filter time constant).
High level noise pick-up in sensor		Low level leads between sensor and transmitter should be shielded.
leads
Transmitter Electronics Failure		Check transmitter STATUS. If a FAILSAFE message is posted, refer to
		Section 6.3.2.1, paragraph A, for troubleshooting procedures.
		If STATUS is OK, select LOOP OVRD (loop override). Verify loop by setting
		output current to 4, 12 and 20 mA (read on Digital Meter or ammeter connected
		to TEST +/- terminals). Carefully observe that the selected currents remain
		steady.
		If current cannot be set or is unstable, replace the Electronics Module. See
		Section 6.5.

6.3.3 Diagnosing a Defective Digital (LCD) Meter

The optional LCD is functionally tested during a transmitter power up or master reset. It is also tested when the MXC TEST program (transmitter self-diagnostics) is executed. The 4 second test activates all the legends and numerical segments of the LCD. However, if the LCD fails the test, the failure is not reported by the self-diagnostics. The LCD must be observed when performing the MXC TEST program to confirm its test.

If the LCD is not functioning correctly, use an on-hand spare LCD to prove the suspect LCD is defective as follows:

September 1995 6-25