Instructions

UM-0085-B09 DT80 Range User Manual Page 378
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CEM20 Temperature Reference
Each CEM20 includes a precision RTD temperature sensor. This is used to measure the reference junction temperature
when thermocouple measurements are performed using the CEM20's analog inputs (see Temperature – Thermocouples
(P315)). This process is automatic and is part of the normal thermocouple measurement process.
A CEM20's internal temperature sensor can be read at any time (for testing purposes) using the nREFT channel, where
n is the CEM number (1-16). So to read the current reference junction temperature for the first CEM20 you would enter
1REFT. (To read the DT80's internal temperature sensor uses the REFT channel – no channel number.)
Troubleshooting
In the event that invalid readings are obtained when reading CEM20 channels, the following troubleshooting procedure
may be helpful.
The key diagnostic aid is the red Sample LED on the CEM20, which will flash when it is commanded to take a
measurement. For example, if you measure input 7 on CEM20 #12 (e.g. by typing 1207V), you should see the red
Sample LED on CEM20 #12 flash, along with the blue Sample LED on the DT80. No other CEM20s should flash their
LED.
 CEM20 Sample LED does not flash
If the appropriate CEM20's Sample LED does not flash when one of its inputs is measured then the most common
explanations are:
• the CEM20 address switch is not set correctly. The CEM20 connected to analog input 1 on the DT80 must be
set to address 1 (switch setting: off, off, off, ON), the CEM20 connected to analog input 2 must be set to
address 2 (switch setting: off, off, ON, off) and so on.
• the power/control cable to the CEM20 is not connected properly. Double check the wiring against the wiring
diagram. Do not confuse the CEM20's power/control input (lower terminal block) and its power/control output
(upper terminal block). Also, when connecting to a DT85, be sure that the 2-way terminal block on the cable is
correctly plugged in to the 3-way terminal block on the DT85 (i.e. the wire should connect to the 12V terminal).
• the CEM20 is not powered. If the CEM20 is powered from the DT80's 12V output then check that the P28
setting is correct; it should be set to 0, 1 or 2. If the CEM20 is powered externally then check that 12V ± 5% is
present.
• there is hardware or cabling problem with one of the "upstream" CEM20s. Try temporarily bypassing them by
disconnecting the cable from CEM20 #1's power/control input and connecting it instead to the CEM20 in
question. Then issue the same measurement command (it is not necessary to change the address of the
CEM20). If this reading is satisfactory then it should be possible by a process of elimination to determine which
CEM20 or power/control cable is faulty.
• you are using a Series 1 DT80. The CEM20 requires a Series 2 unit.
 CEM20 Sample LED flashes, but reading is invalid
Possible explanations are:
• faulty sensor. Check by plugging it into one of the DT80's analog inputs.
• the analog cable of the CEM20 is not connected properly. Double check against the wiring diagram; in particular
check that it is plugged into the correct DT80 channel (a CEM20 set to address n must connect to DT80 analog
input n)
• multiple CEM20s are being selected. Ensure that the Sample LED flashes on one and only one CEM20.
• electrical noise – ensure that the cable shield is connected to a DGND terminal (as shown in the wiring diagram)
or to the DT80 chassis earth point.
• faulty analog input on CEM20 or DT80. Disconnect the CEM20 and connect a sensor directly to the DT80
analog input previously used by the CEM20. This should identify whether the CEM20 or DT80 is at fault.
It may also be helpful to try reading the internal temperature sensor on the CEM20 (e.g. type 12REFT to read the
current temperature of CEM20 #12). This will eliminate any external sensor issues.