Worked Example: Connector Selection Packet

This is what "done" looks like: one interface taken from a loose requirement all the way to a documented, buildable, reviewable packet. It ties together the selection workflow, the rugged-on-a-budget and removable machine module decision paths, and every template under Tools & Templates.

Read this as reasoning, not a shopping list

This example teaches how to decide and document, not what to buy. Part numbers are placeholders like J1, P1, and CONTACT-SIZE16-SKT-EXAMPLE. A real project replaces every placeholder with an exact, datasheet-verified P/N, and sizes every current against the manufacturer's derating curve. No rating below is asserted for you.

Scenario

A small outdoor field robot has a removable sensor/control module. When the module is pulled for service, its harness must disconnect cleanly at the module boundary. The interface must handle:

24 VDC power input to the module
Several low-current discrete I/O lines
One CAN (or RS-485) differential pair
A chassis / shield strategy
Sealed external connection, outdoor splash/dust exposure, moderate vibration
A technician-serviceable harness
No formal MIL-DTL / QPL requirement
A budget-sensitive prototype-to-small-production path

1. Requirements summary

Item	Value / decision	Notes
Subsystem boundary	Robot chassis harness ↔ removable module	One clean disconnect at the module face
Environment	Outdoor, dust + splash, moderate vibration	Target sealed when mated; capped when unmated
Power	24 VDC nominal input	Size current against the contact derating curve — not assumed here
Discrete I/O	Several low-current lines	Direction defined per signal in the pinout
Data	One CAN / RS-485 pair	Twisted pair; bus termination is a system property, not a connector feature
Shield / chassis	Cable shield + chassis-ground strategy	Defined in the pinout and cable notes
Service model	Technician-serviceable, field	Rear-release crimp contacts, hand-tool crimpable
Production posture	Prototype → small production	Low tooling cost, easy sourcing, second source available
Constraints	No MIL/QPL requirement; budget-sensitive	Drives toward sealed automotive, away from mil circulars

2. Connector family candidates

At least three families are worth a look before committing:

Sealed automotive (Deutsch DT / DTM / DTP) — sealed, hand-crimpable, cheap, ubiquitous. See rugged-on-a-budget.
M12 (A-coded + a separate coded connector for CAN) — clean COTS ecosystem, molded cordsets. See industrial sensor.
MIL-DTL-38999 — rugged and configuration-controlled, but overkill here. See the 38999 deep dive.
Industrial rectangular / Han-style — great for serviceable modules, but large for a small robot. See removable machine module.

3. Decision matrix

Qualitative scoring — no fake precision. + favorable, ~ acceptable/depends, − unfavorable for this job.

Candidate	Why it fits	Why it may not fit	Tooling / assembly	Sealing / serviceability	Cost / availability	Decision
Deutsch DT/DTM/DTP	Sealed, cheap, hand-crimpable, rear-serviceable; power + signal covered by one family	Not a mil ecosystem; no EMI backshell ecosystem	`+` low-cost hand tools	`+` IP67/IP68 sealed, rear-release serviceable	`+` cheap, widely stocked, second-sourced	Selected
M12 (A-coded + CAN-coded)	Clean COTS, molded cordsets, sealed	Two+ connectors for this mix; per-connector pin/current limits; more connectors on the panel	`~` cordsets easy; field-wireable fiddly	`+` IP67 sealed when mated	`~` low-cost but more connectors	Backup
MIL-DTL-38999	Rugged, keyed, config-controlled	Cost, tooling, lead time all unjustified with no requirement driving them	`−` positioner/insert tooling	`+` sealed, but far beyond need	`−` expensive, long lead	Rejected
Industrial rectangular / Han	Serviceable, mixed media in one housing	Too large/heavy for a small robot module	`~` insert tooling	`+` serviceable	`~` mid cost, bulky	Rejected

4. Selected architecture

Split into two sealed connectors from the same family: one power, one signal + data. Both Deutsch-class for common tooling.

J1 / P1 — Power (small pin count, larger contacts sized for the 24 VDC feed and its return).
J2 / P2 — Signal + CAN (discrete I/O plus the CAN pair on a smaller-contact housing).

Why split rather than one mixed connector?

Segregation — keeps the 24 VDC power path away from low-level discrete and the CAN pair, reducing coupling and simplifying the shield story.
Serviceability — power and signal harnesses can be built, tested, and replaced independently.
Smaller, cheaper housings — two small sealed connectors are easier to route, seal, and hand-crimp than one dense mixed one.
Failure isolation — one large connector is a single point of failure and a heavy, stiff cable; two smaller ones de-risk both.

The cost is two disconnects instead of one and two caps — acceptable here. If panel space were extremely tight, a single mixed connector with documented power/signal segregation would be the trade the other way.

5. Pinout

Generic signal names; contact sizes are illustrative and must be sized against the datasheet and derating curve.

J1 / P1 — Power

Pin	Signal	Direction	Wire (class)	Shield / twist	Service notes
1	+24 VDC	Into module	Power gauge, sized to load + derating	—	Larger contact size
2	24 V RTN (0 V)	From module	Power gauge, sized to load + derating	—	Larger contact size

J2 / P2 — Signal + CAN

Pin	Signal	Direction	Wire (class)	Shield / twist	Service notes
1	DISCRETE_IN_1	Into module	Signal gauge	—
2	DISCRETE_IN_2	Into module	Signal gauge	—
3	DISCRETE_OUT_1	From module	Signal gauge	—
4	CAN_H	Bidirectional	Twisted pair w/ pin 5	Twisted pair; shielded	Keep pair together end-to-end
5	CAN_L	Bidirectional	Twisted pair w/ pin 4	Twisted pair; shielded
6	SIGNAL_RTN / shield drain	Reference	Signal gauge	Shield drain — bond one end	See cable notes for shield termination

6. BOM checklist

Every line is a real orderable item. Placeholders must become exact P/Ns.

Item	J1/P1 (power)	J2/P2 (signal + CAN)
Connector body (receptacle)	`J1-RCPT-EXAMPLE`	`J2-RCPT-EXAMPLE`
Mating connector (plug)	`P1-PLUG-EXAMPLE`	`P2-PLUG-EXAMPLE`
Contacts (pins/sockets)	`CONTACT-SIZE16-*-EXAMPLE`	`CONTACT-SIZE20-*-EXAMPLE`
Wedgelock / secondary lock (TPA)	as applicable	as applicable
Cavity / sealing plugs	for every unused cavity	for every unused cavity
Rear seal / grommet / strain relief	per family	per family
Dust cap (unmated protection)	`CAP-J1-EXAMPLE`	`CAP-J2-EXAMPLE`
Crimp tool + die/positioner	for contact size	for contact size
Extraction tool	for the retention system	for the retention system
Cable	power cable	shielded cable w/ a twisted pair
Labels / heat-shrink markers	both ends	both ends
Boot / heat-shrink (if used)	as needed	as needed

7. Cable drawing notes

Wire gauge — power conductors sized to the load and the contact derating curve; signal conductors per the contact and signal.
Pair twisting — CAN_H/CAN_L a maintained twisted pair end-to-end; do not split the pair through the connector transition.
Shield termination — system- and frequency-dependent, not a default. This packet assumes a single-point chassis-end bond for the low-frequency CAN/discrete noise concern, and records the strategy, the frequency range of concern, and the rationale in the ICD. One-end vs. both-ends vs. 360° backshell is a per-design decision — see EMI, shielding, and bonding (§5.7).
Cable OD — must fall inside each connector's seal/gland range or the seal and strain relief do not work.
Bend radius — respect the cable's minimum bend radius at the connector exit; pick a straight vs. right-angle backshell/boot accordingly.
Label scheme — both ends of every wire and both connector shells (J1/P1, J2/P2).
Continuity test — point-to-point continuity and correct pinout on every built harness.
Pull-test / inspection — crimp pull-test to the contact spec and a visual crimp inspection criterion.

The template for this is the cable drawing template.

8. ICD entry

Interface name: Module boundary — Power (J1/P1) and Signal+CAN (J2/P2)
Connector role: Receptacles J1/J2 on the module; plugs P1/P2 on the chassis harness
Mating pair: J1↔P1, J2↔P2 — keyed/coded so P1 cannot mate J2
Pinout: per §5 above (source-controlled)
Voltage / current class: 24 VDC nominal; per-contact current sized against the derating curve (verify)
Signal definitions: discrete I/O directions per pinout; CAN_H/CAN_L differential pair
Shield / chassis treatment: single-point chassis-end drain bond (documented assumption for a low-frequency noise concern — strategy, frequency range, and rationale recorded here; revisit per §5.7 if the noise problem changes); module chassis-ground path defined
Environmental assumptions: sealed (target IP67-class) when mated and locked; unmated only when capped (verify the exact family/assembly rating)
Service / cap note: dust caps on both unmated receptacles; rear-release crimp for field repair
Revision control: this ICD and the pinout are rev-controlled; changes go through the interface owner
Source / evidence tracking: every rating in the released version cites its datasheet + revision, the derating basis, and — because this packet is a teaching example — every placeholder is marked example-only (verification status: example). Family-level figures trace to the sourced §3.2 table; nothing here is a verified part rating

Use the ICD template for the full form.

9. Design review checklist

Run the full design review checklist before sign-off.

10. What would change if…

A formal defense / QPL requirement appears → reconsider MIL-DTL-38999 (or 26482 where a bayonet fits); the sealed-automotive choice no longer satisfies the requirement.
Ethernet is added → add a rugged Ethernet path (M12 D/X-coded or sealed RJ45); do not try to run gigabit on the discrete signal connector.
RF / GPS is added → use the RF/GPS/radio path — a coax contact or a separate coax bulkhead, not a spare signal pin.
Current increases significantly → revisit contact size, the derating curve, and possibly a dedicated/split power connector.
Production volume increases → revisit tooling (applicator vs. hand crimp), second sources, keyed variants, and assembly inspection sampling.

Templates for each artifact above live under Tools & Templates. For the family-selection reasoning behind the choice, start at rugged-on-a-budget.

Scenario​

1. Requirements summary​

2. Connector family candidates​

3. Decision matrix​

4. Selected architecture​

5. Pinout​

6. BOM checklist​

7. Cable drawing notes​

8. ICD entry​

9. Design review checklist​

10. What would change if…​