Avionics Bending: Horizontal Situation Indicator - HSI (Analysis and Pinout)

Thanks to some very kind people at Thales (the company that acquired Sextant which acquired SFENA a long time ago) I have been able to do a lot more research around the SFENA ADIs and HSIs that I intend on using for the project.

The instruments I have are from the early 1980s. The design of the ADI and HSI is truly amazing and shows just how much attention to detail and quality went into the engineering of them. The ADI and the HSI are based on ARINC 429 which is by far the most common avionics bus standard in use today (however, it is slowly being replaced by more modern standards).

Both the SFENA N33 (HSI) and N32 (ADI) instruments are based on a rather simple 8bit microprocessor made by RCA … namely the 1802RCA. The instruments are actually somewhat multi-processor systems because they are split into an input processors, handling the ARINC 429 communications, and a central processor handling all of the data transformations and the servo motor control. In order to reduce space needed by the electronics and to reduce the need for expensive resources, many of the functions in the instruments are multiplexed and use the same subsystems.

Specifically on the HSI, there are a number of subsystems that make up the entire electronics complex:

1. Input Processors

2. Central Processing Unit

3. A/D Converter

4. Servo Channels and Controller

5. Pointer Servomechanism Channels

6. Flags and Annunciator Subsystem

7. Magnetic Wheel Drivers

8. Data Output Processor

9. Power Supply and Lighting

The data used by the HSI would have been produced by a number of systems on-board and delivered to the instrument of several ARINC 429 buses. To be exact, the instrument receives data from a total of 8 data buses.

Bus 1 – Primary Attitude and Heading Reference System (AHRS)

Bus 2 – Automatic Flight Control System (AFCS)

Bus 3 – Instrument Landing System (ILS)

Bus 4 – ADF 1

Bus 5 – Flight Management Computer or GPS

Bus 6 – ADF 2

Bus 7 – Reciprocal HSI

Bus 8 – VOR

The above listed buses all feed a number of ARINC 429 labels to the HSI. Here is a list of labels on each of the buses:

Bus 1 – Magnetic Heading (Label 320), Drift (Label 321) and Ground Speed (Label 012)

Bus 2 – Heading Select (Label 101)

Bus 3 – Localizer (Label 173), Glide Slope (Label 174) and QFU (Label 105)

Bus 4 – ADF 1 (Label 162)

Bus 5 – True Heading (Label 314), Drift (Label 321), NAV Course (Label 114), Cross Track Distance (Label 116), NAV Vertical Deviation (Label 117), Ground Speed (Label 012), Distance-To-Go (Label 001), Alert (Label 270) and TO-FROM (also Label 270 but different bit)

Bus 6 – ADF 2 (Label 162)

Bus 7 – MAG (sin, cos recopy) (Label 320) and TRU (sin, cos recopy) (Label 314)

Most of the busses above run ARINC 429 Low Speed (12Kb/s) … however Bus 1 and Bus 7 require the faster 100 Kb/s ARINC 429 bus speed. Besides the ARINC 429 data,. The instrument also has a number of logic inputs that control the flags and other elements. The discrete inputs are:

· Normal / Alernate Switching for AHRS ….. 0V Normal, 28V Alternate

· True-Mag Selection …. 0V MAG, 28V TRUE

· HDG Select/Drift Selection … 0V HDG SEL, 28V DRIFT

· Sensor Number …. 0V … 1, 28V … 2

· Mode Selection … has two ports (pins) to encode the mode

• 0V 0V ….. VOR
• 28V 0V …. ILS
• 28V 28V …. NAV/FMC
• 0V 28V …. NAV/GPS

· HDG SEL / Drift Flag …. 0V Normal, 28V Pointer to 180 degree

· Vertical Deviation Flag …. 0V Normal, 28V Flag Retracted

· Selection of Bus for Drift Angle …. 0V AHRS/IRS, 28V FMC/OMEGA

· Selection of Bus giving Ground Speed …. 0V AHRS, 28V FMC/OMEGA

Here is the J1 connector pin-out for the SFENA N33 HSI:

Pin	SFENA Description	Notes
A	115V 400HZ	POWER INPUT
B	115V 400HZ	POWER INPUT
T	AHRS IRS Normal	ARINC 429 Input
U	AHRS IRS Normal	ARINC 429 Input
X	AHRS IRS Alternate	ARINC 429 Input
Y	AHRS IRS Alternate	ARINC 429 Input
a	Reciprocal HSI	ARINC 429 Input
b	Reciproacl HSI	ARINC 429 Input
M	ILS	ARINC 429 Input
N	ILS	ARINC 429 Input
J	VOR	ARINC 429 Input
K	VOR	ARINC 429 Input
L	ILS/VOR	Switch Input
P	AFCS Controller	ARINC 429 Input
R	AFCS Controller	ARINC 429 Input
DD	True/Magnetic Heading Switch	Switch Input
G	FMC or OMEGA	ARINC 429 Input
H	FMC or OMEGA	ARINC 429 Input
EE	NAV Source Switching	Switch Input
t	ADF 1	ARINC 429 Input
u	ADF 1	ARINC 429 Input
V	ADF 2	ARINC 429 Input
W	ADF 2	ARINC 429 Input
BB	Vertical Deviation	Logic Signal
AA	Heading Select	Logic Signal
FF	Sensor Identifier	Logic Signal
m	Ground Speed Source	Logic Signal
k	Drift Source	Logic Signal
CC	HDG SEL/DRIFT	Logic Signal
z	Reciproacl HSI	ARINC 429 Output
y	Reciproacl HSI	ARINC 429 Output
D	5V Integral Lighting	POWER INPUT
E	5V Integral Lighting	POWER INPUT
g	Validity Output
n	Validity Output
f	Validity Output
p	Comparison Warning Output