Data Line Monitor model X21LM


A simple to use tool for testing and troubleshooting high speed X.21 interfaces, the LEDs instantly show the state of all signals.

(Technical note: X.21 is a specification for a communications interface that uses a DB-15 connector and a set of six V.11 (RS-422) differential signals, each carried on two wires).

  • A complete X.21 line monitor for speeds up to 10Mb
  • Monitors all six X.21 signals
  • Two in-line LED monitors show the signals from the DTE :- Transmit and Control
  • Four in-line LED monitors show the signals from the DCE :- Receive, Indication, Signal Element Timing and Byte Timing
  • Four state monitoring of signals - MARK / SPACE / CLOCKING / NONE
  • Interface connectors: One gold plated female DB-15 type connector, one gold plated male DB-15 connector
  • Interface or battery powered
What's included  

X21LM data line monitor, two AA size batteries, carrying pouch, instruction booklet, 12 months warranty, lifetime support by phone or email

Order code:  X21LM   We regret we can no longer supply this item  
Need something more comprehensive? Click here to learn about our X21 breakout box  
X21LM abbreviated specification & additional information
  • Monitors all active lines:- 2 & 9 (Transmit), 3 & 10 (Control), 4 & 11 (Receive), 5 & 12 (Indication), 6 & 13 (Signal Element Timing), 7 & 14 (Byte Timing) 
  • Four state monitoring of each line: red = SPACE/ON (A-B > +0.3V), green = MARK/OFF (A-B < -0.3V), both = data or clock, neither = signal in transition region (-0.3V < A-B > +0.3V)
  • Suitable for operation to 10Mb/s, LEDs do not dim with increasing speed, the threshold remains at 300mV from DC to 10Mb/s
  • Interface connectors: One female DB-15 type connector, one male DB-15 connector
  • Power: interface power is sufficient for most situations (the power LED indicates the strength of the of the power available), battery powering is possible for extremely weak interfaces. Two AA size batteries (provided) last for over 150 hours
  • Housed in a rigid ABS case
  • Temperature: operating 0 to + 50 deg.C,   storage: -25 to + 70deg.C
  • RoHS compliant. CE marked, made in EU

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                       New Forest Electronics Tel. +44 (0) 1425 650089                 Issue 3.87    23rd April 2021