Breakout box model X21BOB


A comprehensive but easy to use tool for testing and troubleshooting X.21 interfaces. A bank of 15 switches allows any line to be opened or closed, the LEDs instantly show the state of the incoming signals, any line can be patched to any other using the jumper leads supplied.

(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 fully featured, high speed (10 Mb) X.21 test tool
  • Monitors all X.21 signal lines, tests voltage levels and ground potential difference
  • Self explanatory face plate design for ease of use
  • 15 slide switches, to open or close any line
  • The in-line LED monitors to the left of the switches show the two signals from the DTE:- Transmit and Control
  • The in-line LED monitors to the right of the switches show the four signals from the DCE :- Receive, Indication, Signal Element Timing and Byte Timing
  • Four state monitoring of signals: red = SPACE/ON, green = MARK/OFF, both = clock or data, neither = no signal
  • Patch any line to any other line, or force signal levels, using the jumper leads supplied
  • Monitor/trap facility
  • Ground potential difference test
  • Two dual gender ribbon cable adapters included, for connectivity in any type of DB-15 configuration
  • Battery powered (batteries included), or can be used with an external 9V supply

(Click for larger picture)


X21BOB breakout box, two dual gender ribbon cable adapters, set of jumper leads, carrying pouch for everything supplied, instruction booklet, 12 months warranty, lifetime support by phone or email

Order code: X21BOB   We regret to say that from 30th September 2010 this product is no longer available from our supplier. We will update this page should the situation change.  
Need something simpler? Click here to learn about our X21 line monitor  
X21BOB abbreviated specification & additional information
  • Number of breakout switches: 15
  • Number of probe points/sockets: 30 (identified by the labelled LED monitors next to each position)
  • Total number of monitors (2 LEDs each): 7 in-line (including G.P.D. pair), 2 unassigned
  • 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: 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
  • One unassigned monitor/trap for signal level and pulse detection, accessed by patching jumper leads from required lines
  • Signal simulation by patching from voltage source sockets above the switches
  • Interface connectors: One gold plated female DB-15 type connector on the left-hand side of the box, one male DB-15 connector on the right-hand side
  • Two dual gender ribbon cable adapters included, for connectivity in any type of DB-15 configuration
  • Power: Isolated 1.5 volt AAA cell (included) for balanced line LEDs, 9 volt battery (included) for general operation (life over 20 hours). May also be powered by an external 9 volt (stabilised, 100mA) mains adapter (not included)
  • Dimensions: (WxHxD) 105 x 150 x 20 mm (4.1 x 5.9 x 0.8 inches)
  • Weight: 295 g (10.4 oz)
  • Temperature: operating = 0 to + 50 deg.C (32 to 122 deg.F)
  • 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