From dataTaker Wiki (FAQ)
CEM20 Channel Expansion Module for the dataTaker DT80 range of data loggers frequently asked questions.
What is a CEM20?
The CEM20 is a purpose designed channel expansion module for increasing the channel capacity of the dataTaker DT80 range of data loggers (Series 2 only). Each CEM20 allows 20 universal analog channels to be connected to 1 universal analog channel of the data logger. This effectively expands the maximum analog input capacity of the DT80 to 100 universal analog channels and the DT85 to 300 universal analog channels. Product Details and Data Sheet
What are the input restrictions?
The CEM20 does not restrict the type of analog inputs that can used, if the input type can be connected to the universal analog channel of the data logger then it can also be connected via a CEM20.
Can the CEM20 be used with a DT80 range series 1 data logger?
No, the CEM20 only works with Series 2 & 3 models of the DT80 range of data loggers.
Can the CEM20 be used with a DT81/82?
No, the CEM20 does not work with the DT81/82 (Series 1, 2 or 3).
How is the CEM20 connected to the data logger?
The CEM20 uses 1 (4 wire shielded cable) connection for the analog connection to the data logger and 1 (4 wire shielded cable) connection for power and control.
How far away from the data logger can I mount the CEM20?
The CEM20 is connected to the logger in a "daisy chain" arrangement. The total chain length must not exceed 100m. In other words, the combined distance between each CEM20 in a chain of CEM20s, plus the distance from the logger to the first CEM20, must be no more than 100m.
For example, you could have a logger with two CEM20s, the first one mounted 20m from the logger and the other mounted up to a further 80m away from the first CEM20.
The limiting factors for cable length are resistance and capacitance of the wires. The power wires of the cable must not exceed 12 ohms and the Gnd to Data and Gnd to Clock capacitance must not exceed 20nF. When measuring resistance, the resistance of all cables in the "daisy chain" are added together. In practice, the resistance of a typical 4 core signal cable will limit the total cable length to about 100m.
Care should be taken when connecting cable shields. For the power/control cable, the shield should only be connected at the loggers DGND point. If connecting more than one CEM20 to logger then tie the shields together at each CEM20, rather than connecting to the GND terminal at each CEM20. This will ensure a single ground connection for the shield covering all CEM20 power/control signals and thus avoid any potential ground loops. For the analog signal cable, the shield should only be connected to the DGND terminal or Earth point at the logger end.
IMPORTANT: Always minimise the distances between the logger and any CEM20s to the shortest practical distance to minimise noise and signal losses.
Note: the distance specification was formally 500mm between each CEM20 and the logger. This specification has changed and is compatible with all exisiting CEM20 units.
Do I need a separate power supply for the CEM20?
No. The CEM20 is usually powered from the data logger 12V output. The logger can provide enough power up to the maximum number of CEM20s that can be connected (15 for a DT85). You can use a separate supply if you wish as long as it meets the voltage and current requirements (12Vdc+-5% 30mA).
How much power does the CEM20 use?
The CEM20 will draw ~30mA while sampling (60mS per sample). It draws ~1mA if powered but not sampling and zero if not powered. The default behavior of the logger is to turn off the 12Vdc supply when not sampling, so any attached CEM20s are turned off when no measurements are occurring. The DT80 range user's manual includes examples on calculating power consumption for systems that include the CEM20.
How do I refer to channels located on the CEM20?
The channel number is used to indicate specific channels.
The CEM20 address is in two parts XXYY where XX is the CEM20 address as set on the DIP switches on the end of the CEM20. This address must match the DT80 range channel number the CEM20 is attached and the second part YY is the channel address on the CEM20.
For example; With the DIP switch set to 1 the CEM20 will attach to Channel 1 on the dataTaker and the first analog channel on the CEM20 will be 01.
Each CEM20 address has 99 channels reserved for its use, though only the first 20 are actually used. For example, channels 101-199 refer to channels on CEM20 address number 1. Channels 1201-1299 refer to channels on CEM20 address number 12.