User's Manual

TT21 Transponder Installation Manual 19 January 2009

00560-00 Issue AA

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signals, called A and B, both of which must be connected for communication to work.

TMAP A and B lines on the controller should be connected to the corresponding A and B lines of

either TMAP1 or TMAP2 on the TT21.

5.8.2 Altitude Out

The TC20 incorporates an altitude encoder. Certain GPS receivers can benefit from having altitude

information supplied to them. The TC20 reports pressure altitude on this pin as an RS232 serial data

format, at 9600 bps, using the format commonly called “Icarus” or “Garmin” format.

5.8.3 Remote ON

This output is connected directly to the Power/Mode switch on the TC20, and should be connected to

the Power ON discrete input of the TT21.

5.8.4 Power

The TC20 uses 6.5 volts which is available from the TT21 transponder. This input should NOT be

connected to aircraft power.

5.9 D Connector Crimp Terminals

The D type connector contacts should be wired with wire of 20-24 AWG. The contacts are compatible

with a wide range of crimp tools.

Once crimped, the contacts should be slotted into the rear of the connector shell. Push the contact in

until the retaining tab clicks into place. Tug gently to confirm the contact is locked in place.

5.10 Wiring Considerations

The connection from the TT21 transponder to the TC20 uses a minimum of six (6) signal lines; the

TMAP pair, the Power and Ground pair, and the Remote On discrete line plus associated ground line.

In a certified installation the normal wire choice would be Tefzel hook-up wire. Wire of 20 AWG is

more than adequate for the task; in installations where weight is an issue, wire of 22 or 24 AWG can

also be used. Where lighter wires than 20 AWG are used the individual wires should be laced together

for support.

The TT21/TC20 was tested and certified using unshielded, untwisted wiring, and that is sufficient for a

certified installation. There may however be technical benefits of improved electromagnetic emissions

and susceptibility to and from the transponder system if the two wires of the TMAP pair are lightly

twisted together – one twist per 1 to 2 inches is appropriate. This may reduce interference and break-

through on adjacent audio wiring if it is not possible to route them separately. For tidiness and

consistency the other pairs in the bundle can also be twisted, but there will be no particular difference

in behaviour.

The distance between the TT21 transponder and the TC20 controller is limited by the impedance of the

wire between them. The TC20 is powered from the TT21, not from aircraft power, and therefore the

acceptable voltage drop in the power line is what limits the distance. The TC20 needs an impedance of

less than 0.5 ohm in the power line for satisfactory operation. The following table gives guidelines for

typical aircraft hook-up wire. Note that different brands may vary – check your supplier for details.

Gauge Ohm/k

Length for 0.5

Ohm

AWG

35 14.2 m

22 64 7.8 m

Trig Avionics Limited Page 11