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Section 10.0 Known Compatibility Issues
10.1 Honeywell KT 73 Transponder with Serial Altitude Data Input
The KT 73 must be configured to accept serial altitude data on pin 7 of the main connector, and
software configured for High Resolution M (IIMorrow). 1200bps, 7 data bits, 1 stop bit and odd
parity. The Trans-Cal SSD120-(XX)N-RS5 must be software configured to transmit the correct
serial data protocol to the Bendix/King KT 73 transponder as described in §5.0 of this manual.
10.2 Narco AT5A, AT6A, AT-50 and AT-50A Installations and SSD120-(XX)N Mod.1
The Narco AT-5A, AT-6A, AT-50 or AT-50A transponder will not accept data from the SSD120-
(XX)N-RS5 Altitude Digitizer. A modification to remove the output decoupling capacitors is
required and the unit may be ordered from the factory with this modification. Order Model Number
SSD120-(XX)N with Mod. 1. Please note! This modification may NOT be performed in the
field. Please also note! The Narco AT-50 and earlier transponder models require a modification
to the strobe circuit before they will function correctly with any altitude encoder. This modification
is outlined in Narco Service Bulletin AT-50A-5.
10.3 "Mod. 1" Installations
SSD120-(XX)N-RS5 Mod.1 Altitude digitizers lack decoupling capacitors on the ICAO altitude
data line outputs to enable operation with older Narco transponders. When installing a Mod. 1 unit
in an aircraft, and particularly when installing in composite structured aircraft, great care
should be taken to insure that the digitizer is located away from RF emitting devices and fields.
The interconnecting data harness must be shielded and properly grounded. Additional shielding
around the digitizer may be required to prevent stray RFI from disrupting the digitizer's analog
signal sensing electronics.
10.4 King KT-75
The King KT-75/75R uses the old RTL (resistor transistor logic) pulling up to about 3 volts;
consequently the open collectors of the SSD120-(XX)N will not pull the signal past the KT-75
logic threshold.
10.5 S-Tec (Collins) TDR950
The TDR950 must be powered-up first, or the SSD120-(XX)N must be diode isolated to prevent
the TDR 950 from invalidating the encoder data.
All diodes are type 1N4454 (CPN 353-3741-010).
10.6 Trans-Cal SSD120-(XX)N Backwards Compatibility
All Model SSD120-(XX)N-XXXX are pin-for-pin replacements for all Model SSD120-(XX) and
D120-P2 T, with ONE exception. The older SSD120-(XX) utilized a 28V heater ground on pin 1 of
the D-Subminiature connector. Pin 1 is the D4 data bit on the SSD120-(XX)N models. Rewire the
harness appropriately, if D4 is an active bit. No action is required if D4 is unused. All Model
SSD120-(XX)N-XXXX are pin-for-pin replacements for all Model SSD120-(XX)A-XXXX.
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