HDP20 pin assignments - finalisation

So I am going to insert the pins, after swapping the engine control (start, magneto, etc) flaps and trim, to the front connector to get them far away from the audio stuff and because the engine control lines are a little short. I am also not inserting the magnetometer lines, deciding to route the magnetometer cable (with its own plug) all the way to the panel. 4 pins are freed up, and I do not have to address the question of whether or not the +12 auxillary power is actually required by the magnetometer. I did not include it. Online discussion says it is not included, but I have learned to distrust such forum talk, or worse, AI. Little is gained by including the magnetometer into the midsection interconnect, and by keeping it separate will facilitate debugging of any subsequent network issues, by enabling the servos to be fully separated from the rest of the network including the magnetometer. I also don't have to redo a bunch of cable splices. In addition, the backup EFIS, likely a Kanardia Horis, will have its own CAN bus-based remote magnetometer. This will be also connected by a cable which will pass directly from the panel though the midsection, also bypassing the interconnect. For this particular cable, there are two reasons for bypassing the interconnect. The first is that the CAN bus has a controlled impedance transmission line, which could be disrupted by being passed through the interconnect. Second, there are just not enough spare wires. Some pins could be liberated by consolidating a shield or two, or a ground, but the payoff does not seem so great merely in the pursuit of "purity" (i.e. everything passing thought the interconnect). Hell, the pitot static lines don't, so I am not going to get bent out of shape about two rather than one electrical cables not passing though the interconnect.

I also added 2 lines for the fuel pump, which I had forgotten about. I will use shielded tefzel for this, so 3 pins required

I have also swapped the backup battery input and outputs between the connectors

So the revised scheme is this:

Cable jackets correspond to subheading names

Individula wire labels correspond to to names alongside the bracketed pin numbers

Connector A (front)

Taxi lights

• 2 size 16 pins, 
• pos (4)
• neg+shld aggregated (1)

IBBS battery (shared with B connector)

• 3 size 16 pins (13A rating)
• backup out (4 20AWG wires from DB15 condensed into 1 pin) (34)
• backup gnd (3 20AWG wires from DB15 wires condensed into 1 pin) (6)
• pass-thru (3 20AWG wires from DB15 wires condensed into 1 pin) (5)
• this connection may not actually be used, if it is decided that the pass-through function of the backup battery is not utilised

• 2 size 20 pins
• charge/sense (41)
• backup master (13)

The backup battery master is only one of two lines that will be live even if the main bus is dead. And this live output will be on a visible pin. By placing the backup power master on the same connector, then if this connector is decoupled the backup battery master pin will be dead

Tail ADS B-In

• 2 size 20 pins
• serial rx (38)
• serial tx (37)

Transponder

• 2 size 20 pins
• serial rx (24)
• serial tx (23)

Tail ADS B-in/Transponder

• 2 size 20 pins
• pwr (21)
• gnd (22)

Stick

• 5 size 20 pins (AP disconnect omitted)
• lh trim up (32)
• lh trim down (18)
• rh trim up (19)
• rh trim down (7)
• stick gnd (31)

Servos

• 6 size 20 pins
• pos (10)
• neg (11)
• net 1a (9)
• net 1b (3)
• net 2a (8)
• net 2b (???)

Ignition

• 4 size 20 pins(purple band)
• ignition gnd A (42)
• ignition gnd B (15)
• common shield (connected to intake manifold) (27)
• separately run 18AWG ground return (redundancy wire for common shield) (28)

Start contactor

• 1 size 20 pin (purple band)
• start (pull up) (12)

Master contactor

• 1 size 20 pin (purple band)
• master (39)

If this connector is unplugged the pin will be at the battery voltage. But shorting protection is not required as this will be the bottom end of the master contactor coil. This pin is meant to be grounded to close the master contactor

Regulator enable

• 1 size 20 pin
• reg enable (25)

Alternator undervoltage/CSF indicator

• no longer taken through midsection interconnect, routed direct to EMS220

Battery fault indicator

• no longer taken through midsection interconnect, routed direct to EMS220

Flap control

• 5 size 20 pins
• pos (45)
• gnd (30)
• flap up (44)
• flap down (29)
• airspeed (16)

EFIS main power

• the supply to the EFIS, limited by its CB
• high side for the EMS220 warning light output (low side is EMS220 warning light output)
• the main bus voltage monitored by the EMS220
• 1 size 20 pin
• main voltage (40)

EMS220 warning light output

• 1 size 20 pin
• ext alarm (26)

EMS gnd reference

• 1 size 20 pin
• EMS gnd ref (43)

Headset power

• 1 size 20 pin
• headset pwr (17)
• this might have its own CB rather than sharing the COM radio power CB, for added system safety

Fuel pump

• 2 size 20 pins (purple band)
• pos (47)
• neg + shld aggregated (33)

Trim

• 2 size 20 pins 
• red (35)
• grn (20)

A subtotal

Keying pins: 3 size 20

Signal/pwr pins: 39 size 20, 5 size 16

So all size 16 pins used, 0 spare size 20

Connector B (aft)

Landing lights

• 3 size 16 pins
• lh pos (4)
• rh pos (6)
• neg + shld aggregated (5)

Nav/strobe lights

• 3 size 20 pins (purple band)
• nav pos (11)
• strb pos (10)
• neg + shld aggregated (3)

USB

• 2 size 20 pins (purple band)
• pos (35)
• neg + shld aggregated (36)

GPS2020

• 4 size 20 pins
• plus8v (19)
• gnd (17)
• serial rx (2)
• serial tx (7)

Flarm fusion

• 3 size 20 pins
• serial tx (12)
• pwr (39)
• gnd (25)

COM radio

• 5 size 20 pins
• gnd (14)
• pwr (26)
• enable (15)
• data rx (13)
• data tx (40)

Intercom

• 6 size 20 pins,
• audio left (44)
• audio right (17)
• audio gnd (45)
• pwr (18)
• gnd (30)
• dim (46)

Left wing OAT

• 3 size 20 pins
• signal (27)
• pwr (42)
• gnd (28)

Right wing OAT

• 2 size 20 pins
• (43)
• (16)

ELT beacon

• 4 size 20 pins
• elt rx (9)
• rmt sw (21)
• ext on (20)
• gnd+shld aggregated (8)

Overhead lights

• 3 size 20 pins,
• white gnd (33)
• red gnd (47)
• plus 12-16V (32)
• plus 12-16V line is connected to PWM dimmer control output
• white and red gnds are switch-connected to gnd bus

Compass light

• 2 size 20 pins
• pos (38)
• connected to overhead lights PWM dimmer output OR input, all on "interior lighting" CB depending on whether compass light is respectively dimmable or not
• neg (24)
• connected to gnd bus (compass light is incandescent so polarity does not matter)

B subtotal

Keying pins: 3 size 20

Signal/pwr pins: 37 size 20, 3 size 16

So 2 size 16 pins spare, 2 size 20 pin spare (pins 23, 37)

Wires routed from firewall forward and backup battery direct to EMS220

• Alternator undervoltage/CSF indicator. If routed to general purpose input will need a 10k series resistor, if routed to an enhanced general purpose input no resistor needed
• Battery fault indicator
• Backup battery low volt
• Backup battery voltage

The main bus voltage monitored by the EMS220 is the EFIS supply voltage, now routed through the interconnect

And the EMS220 ext fault line is routed back to the midsection interconnect

14 Apr 2026

I have aggregated the shields of the 18AWG looms on interconnect B. The size 20 shield pin is rated to carry a fault current from one of the three circuits, in the event of a short from the pos line on any one of the looms to its shielding. This is a compromise. Previously each loom had its own shield path of which the associated size 20 (or 16) pin is rated to carry the full CB rated load associated with a short from the pos line to the shielding. I will give this more thought.

At this stage the near strict star-pointing of gnd lines has been preserved, although departing from this would be alternate means of recovering signals, as well as reducing the number of keying pins

15 Apr 2026

Above scheme dumped. Instead, I combine the neg and shields before passing through the midsection interconnect. In most cases I will be dumping shielding for the final run between the interconnect and the panel. This frees up some pins for added flexibility. I might separate out the tail beacon strobe function from the wingtip strobes. I have also increased the flexibility of the compass lighting, allowing it to be powered either from a interior lighting circuit main bus or from the dimming PWM output. I have also combined the magneto ground path with the shield at the interconnect, allowing the use of a single pin. Both the magneto and ELT will employ shielding for the final run to the panel

19 Apr 2026

Magneto ground return kept separate from shield at present, for added redundancy against wire breakage

Pitot heat active signal restored, since the EMS220 now monitors this

21 Apr 2026

Dump the pitot heat active signal, restore the low voltage detect from the IBBS, based on a revised understanding that this signal warns of a low INPUT voltage to the IBBS. This may occur, and be otherwise undetected, if the CB to the IBBS is popped and not noticed. So overall, the pitot heat wiring can probably be removed

25 Apr 2026

Add a headset power line

Pitot heat cable dumped

Add wires for compass light

28 Apr 2026

Assign wires to HDP20 pin locations

30 Apr 2026

Update documentation with pin locations