SDM8X50 8-Channel 50 Ohm Coaxial Multiplexer for TDR Systems
Hermetically Sealed Long-Lasting Relays
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The SDM8X50 is a 50 ohm, coaxial, 8:1 multiplexer used in a Campbell Scientific time-domain reflectometer system. It consists of a surge-protected multiplexer circuit board enclosed in a metal housing and a separate strain-relief bracket for the coaxial cables. Both the multiplexer housing and strain relief bracket have holes drilled at a 1 in. spacing. This enables you to mount the SDM8X50 to a wall or attach it to the backplate of a user-supplied enclosure or Campbell Scientific enclosure.

When you purchase the SDM8X50 with the –E option, a 10 x 12 x 4.5 in. environmental enclosure and enclosure supply kit are included. Other compatible Campbell Scientific enclosures that you may purchase separately include the ENCTDR100, ENC12/14, ENC14/16, and ENC16/18

Benefits and Features

  • Longer lasting relays; 20 times more operations than SDMX50 relays
  • Non-latching relays provide better surge protection
  • Hermetically sealed relays are more stable and do not become coated with a non-conducting film
  • Quiet operation



Operating Temperature Range -40° to +55°C
Multiplexer Housing Dimensions 24.9 x 12.2 x 4.6 cm (9.8 x 4.8 x 1.8 in.) with mounts
Strain Relief Bracket Dimensions 20.3 x 4.3 x 1.3 cm (8.0 x 1.7 x 0.5 in.)
Multiplexer Weight 590 g (1.3 lb)

Power Requirements

Input Power 12 Vdc
Quiescent Current Drain < 1 mA
Current Drain during Switching ~ 90 mA (All multiplexers of the same level switch simultaneously for less than 1 s.)
Relay Contact Life Expectancy 100 x 106 operations (at 5 V and 10 mA)

Enclosure (-E option)

Outside Dimensions 40.4 x 29.2 x 17.5 cm 15.9 x 11.5 x 6.9 in.) with mounts
Inside Dimensions 25.4 x 30.4 x 11.4 cm (10 x 12 x 4.5 in.)
Weight 4.1 kg (9.0 lb)


Please note: The following shows notable compatibility information. It is not a comprehensive list of all compatible products.


Product Compatible Note
CR1000 (retired)
CR1000 (retired)

Mounting Equipment

Product Compatible Note

Additional Compatibility Information

Enclosure Considerations

When purchased with the –E option, a 10 x 12 x 4.5 in. environmental enclosure and enclosure supply kit are included with the SDM8X50.

FAQs for

Number of FAQs related to SDM8X50: 3

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  1. The hermetically sealed relays used in the SDM8X50 are much quieter than the relays used with the retired SDMX50-series multiplexers. Although listening for the clicking on the SDMX50-series multiplexers was a common method to verify that the multiplexer was powered and communicating with the data logger, that test does not work for the SDM8X50.

  2. The SDM8X50 is the same as the SDMX50SP in form, fit, and function. When purchased with the –E enclosure option, a 10 x 12 in. environmental enclosure is included, making it functionally the same as the SDMX50. There is no comparable replacement for the SDMX50LP, which was the multiplexer and backplate used with the SDMX50 but sold separately for installation in a user-supplied enclosure.

    The primary difference between the SDM8X50 and the SDMX50-series multiplexers is the electromechanical relays. The SDMX50-series products used latching relays with an expected lifespan of 5 x 106 operations. Because these relays were always latched, there was always an electrical pathway back to the TDR100 and an increased risk of damage from an electrical surge. The SDM8X50 uses non-latching, hermetically sealed relays with a lifespan of 100 x 106 operations. Additionally, relays have been added to the ground channels to electrically isolate them from each other and reduce interference. Thus, the SDM8X50 has improved surge protection, quieter operation, and a longer lifespan than the SDMX50-series multiplexers.

  3. The PC-TDR software is compatible with the SDM8X50 multiplexer, but the non-latching relays may cause issues. When PC-TDR (version 2.08 and lower) advances to a multiplexer terminal, it sends one command to the multiplexer. With the older SDMX50-series multiplexers, one command was sufficient because the relays latched. The non-latching relays on the SDM8X50 release 30 seconds after the command is sent, which results in a loss of connection between the sensor on that terminal and the TDR100. To latch again for a good reading, it is necessary to manually force PC-TDR to go to a different SDM8X50 terminal and then back to the terminal of interest. The continuous waveform function should not be used with the SDM8X50. These known issues will be addressed in the next version of the PC-TDR software.

Listed Under

Common Accessory for the following products:
This Product Replaced