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Where the boxes really come into their own is when they are provided
with a patching capability. The twenty five circuits on both sides of the
isolation switches are made available on the control panel, and wires are
provided to allow connections from any circuit to any other.

Clock circuits are a difficulty with Breakout Boxes because they oscillate
too quickly for the lights to come on. Many boxes therefore feature a
“pulse extender” which allows the oscillation to be seen.

Some boxes have lights which can show two colours thus allowing the
state of the circuit to be seen: red=one/off, green=zero/on, both
off=transition. Simpler boxes have a single colour which can only show
limited information.

The following example shows the use of a Breakout Box to create a
connection between asynchronous devices of unknown capabilities and
requirements. Suppose a printer is being attached to a computer as shown
in Fig. 7.35.

Fig. 7.35 Breakout Box inser ted in DTE-to-DCE connection

The first thing to do is to adjust the switches on the box so that all
circuits are disconnected except for Signal Ground and Protective Ground.
Pin 2 of the computer side is then patched to the pulse extender. Fig. 7.36
shows what the box may look like.

With pin 2 connected to the pulse extender, we make the computer
output some data. If the light on the extender now comes on, then we
know that the computer outputs on pin 2 so is probably wired as a DTE.
If the light does not come on, the extender can be connected to pin 3 of
the computer to check that it is a DCE.

If the extender light does not indicate data in either case then the
computer needs some of its other circuits to be held in an on state. The
box will have an “on” voltage available which should be connected as
follows:

• Firstly, to pins 5, 6 and 8 which in the case of a DTE are inputs
to CTS, DSR, and DCD loads. The extender is connected to pin
2 which is XMT output from a DTE.

• If no data is observed then connect the “on” to pins 4 and 20
which in the case of a DCE are inputs to RTS and DTR load. The
extender is connected to pin 3 which is RCV output for a DCE.

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