Annex P (informative) — Rationale

Why each requirement exists, one line each: the failure it prevents, the fit it secures, or where its number comes from. Generated from the rationale field of data/requirements.json by tools/requirements.py. The requirement itself is in its clause and in the register; this annex adds no requirement.

5.1 Rack dimensions

ID	Why
RACK-001	Every width in a row is a multiple of the HP, so quoting the IEC value lets panels and rails from different makers line up hole for hole.
RACK-002	The rack unit sets the 3U and 1U heights that panels, rails, and 19-inch rack frames share, so quoting IEC keeps them all on one grid.
RACK-003	Meeting the IEC flange, tab, aperture, and row figures lets a rack-mount frame bolt into any 19-inch rack and hold a full 84 HP row.

5.2 Modules

ID	Why
PANEL-001	These are the only two panel heights EIS defines; a module of any other height fits no conforming row.
PANEL-002	A panel at or under its nominal width lets neighbors sit side by side without binding, so N HP of panels always fits an N HP row.
PANEL-003	A buyer compares these two figures with a row's published width and clear depth to know the module fits before buying it.
PANEL-004	The mounting-screw length window is computed for panels up to 2.0 mm; a thicker panel leaves too few threads engaged in the insert.
PANEL-005	One patch connector across all modules means any patch cable joins any output to any input, whoever made the modules.
PANEL-006	Three rack units less 4.85 mm of clearance, 128.5 mm lets the panel sit inside the 132.55 mm IEC aperture between the rails.
PANEL-007	Matching the established Intellijel 1U format, 4.80 mm under one rack unit, lets 1U modules fit the 1U rows already in use.
PANEL-008	A fixed inset from each edge puts every panel's holes on the same line as the rail inserts, whatever the maker.
PANEL-009	The span is the panel height less twice the 3.0 mm inset, and the rails must match it or the panel holes miss the inserts.
PANEL-010	A 3.2 mm hole clears the 3.0 mm M3 screw with room for small position errors between panel and insert; a short slot along the rail takes up rail position error without letting the panel drop.
PANEL-011	Offsetting holes by half an HP from the nominal edge puts every hole on an insert hole, whatever the panel width and wherever it sits.
PANEL-012	(128.5 mm − 110 mm) / 2 is 9.25 mm, so the usual 110 mm board fits between rails that cover no more of the panel.
PANEL-013	A panel drawn to a nominal figure without a tolerance cannot be checked or rejected; one stated tolerance policy makes every mechanical figure measurable the same way.
PANEL-014	The screw length window keeps a screw's tip off the cavity floor through a 1.6 mm panel; an enclosure's screws can bottom out through a thinner one, so the thin panel brings screws that fit.

5.3 Rails, inserts, and mounting screws

ID	Why
RAIL-001	Insert holes one HP apart line up with panel holes placed on the HP grid, so a panel screws down at any position in the row.
RAIL-002	N panels at their full nominal width need N × 5.08 mm of rail, so a shorter rail cannot hold the row's published width.
RAIL-003	The insert thread has to take the M3 mounting screw that every module hole is drilled to clear.
RAIL-004	An insert three 0.5 mm pitches thick gives the screw the three full threads RAIL-008 needs to hold a panel without stripping.
RAIL-005	M3 is the thread of nearly every Eurorack case on sale and of the screws modules ship with, so one screw fits any row.
RAIL-006	Quoting the ISO 261 coarse pitch means any off-the-shelf M3 screw threads into any conforming insert.
RAIL-007	Quoting the ISO 261 diameter fixes the size that the 3.2 mm panel hole and the rail throat are made to clear.
RAIL-008	Three engaged threads through the thickest allowed panel keep a screw from stripping out as cables are plugged and pulled.
RAIL-009	The lower bound gives three threads through a 2.0 mm panel, and the upper keeps the tip off the cavity floor through a 1.6 mm panel.
RAIL-010	The narrowest catalog rail face reaches 3.7 mm past the hole center (Vector T-strut, Annex A), so a head up to 7.0 mm across bears on the panel and the rail face on every catalog rail; it admits an ISO 7045 pan head and the common 7 mm M3 washer.
RAIL-011	The right screw length depends on the rail's seat and cavity depth, which only the enclosure maker knows.

5.4 Enclosures

ID	Why
ENC-001	A buyer compares these figures with a module's published width and depth to know it fits the row before buying.

6.1 Power architecture

ID	Why
PWR-001	These figures are what an installer sums against a source's and a distribution product's ratings, and what a buyer sizes a DC adapter from; stating 0 shows a rail was not forgotten.
PWR-002	An installer checks a row's summed currents against the source and each outlet before adding a module (ASM-001); outlets can be rated below their rails, and traces and connectors that carry more than they are rated for overheat.
PWR-003	The installer connects only one of a product's alternative sources (ASM-003), so the product has to run from any one of them alone; entries that feed different circuits, such as PWR-A6 for the analog rails and the bus for logic, are not alternatives.
PWR-004	A module that drives a rail fights its source's regulator and can back-power other products when the source is off.
PWR-005	Three terminal types cover every supply and bus board sold, and naming them lets a harness from one maker land on a board from another without an adapter.
PWR-006	A row's rails and its 0 V carry every module's current through one or two conductors; the gauges keep the drop and the heating small, the temperature rating suits a closed case, and a crimp holds where a soldered strand fractures.
PWR-007	The 0 V conductor returns the current of every rail at once, so it is never the thinner one.
PWR-008	A bond through paint or a loose terminal is a bond only until it corrodes; a toothed washer on bare metal keeps it low impedance for the life of the case.

6.2 Power supplies and power converters

ID	Why
SUP-001	A ripple figure taken one stated way lets a buyer compare supplies and pick a quiet one for video or audio work.
SUP-002	A supply connected backwards reverses every rail on the bus and can damage every module in the row at once.
SUP-003	Loose tabs and terminals cannot be keyed, so a clear rail marking is the only guard against wiring a rail to the wrong place.
SUP-004	A short from a miswired ribbon or bus board is a routine fault, and the supply has to outlast it until someone finds it.
SUP-005	Op-amps and polarized capacitors on modules tolerate little reverse voltage, so a diode drop is the most any rail may reverse, in any sequence.
SUP-006	Parts on modules are chosen for 12 V rails, and a large start-up overshoot eats the voltage margin they were sized with.
SUP-007	A 150 mV ceiling gives modules a worst case to design against, at a level most open-frame switching supplies already meet.
SUP-008	A linear converter turns the difference between its input and output into heat in the case, and it adds to the depth of the module it sits behind; the installer budgets both (Annex O, ENC-001).
SUP-009	A case supply that gives a DC rail or the analog rails directly saves a brick or a converter per module, and naming the domain keeps its outlets under the rules every product of that domain expects.

6.3 Grounding and 0 V

ID	Why
GND-001	The 0 V pins share the return current of all three bus rails, and using only some crowds it onto fewer contacts; with the DC rail's −V on the same 0 V, a product keeps one reference for its signals whichever entry is connected.
GND-002	One 0 V back to the supply terminal gives every module in the row the same reference and a defined path for return current.
GND-003	A system wants one bond between 0 V and protective earth; a second bond lets mains-frequency current flow in 0 V as hum.

6.4 The bus

ID	Why
BUS-001	Every module may count on ±12 V at its header, while +5 V stays optional because many modules regulate their own.
BUS-002	One header form, the 2.54 mm IDC header of the Doepfer bus, lets any power ribbon mate any module and any bus board.
BUS-003	Ribbons and bus boards made to a different pin order put +12 V on a module's 0 V pins; one pinout prevents that.
BUS-004	A marked conductor tied to −12 V lets an installer orient a ribbon by eye at both ends, the only guide on an unshrouded header.
BUS-005	A key in one fixed position lets a keyed socket enter a shrouded header only the right way round, whoever made each part.
BUS-006	These are the only rails a conforming bus delivers, so a module that needs any other rail cannot run from a conforming row.
BUS-007	Pins 11 to 16 carry nothing such a module uses, so the smaller header saves board space without losing any connection.
BUS-008	Two IDC contacts per rail at about 1 A each set the 2 A ceiling for current through one header.
BUS-009	A reversed ribbon is the commonest field fault; surviving it keeps one mistake from destroying a module.
BUS-010	A keyed cable makes the shroud do its job, so the ribbon that ships with a module cannot be fitted backwards into a shrouded header.
BUS-011	Expander and link headers often share the power header's form, and a power ribbon fitted to one puts ±12 V on its signal pins.
BUS-019	The marked conductor of a ribbon can only be matched to the header when the board says which end is −12 V; on an unshrouded header the marking is the only guide.

6.5 Bus boards

ID	Why
BUS-012	A header carries each rail on two IDC contacts of about 1 A, the same limit modules are held to under BUS-008.
BUS-013	A split lets one long bus board carry two separate CV and gate lines, each with its own transmitter.
BUS-014	A crossed link would send pitch CV to gate listeners and gate pulses to oscillators on the next board.
BUS-015	The voltage drop of a longer or thinner ribbon at the 2 A header limit eats into the rails the module receives; 28 AWG is the conductor of standard 1.27 mm pitch ribbon.
BUS-020	One board behind the panel can carry the bus, the DC rail, the analog rails, the video reference tree, and the case's USB hub; naming each feature keeps every outlet and port on it under the rule written for that interface.

6.6 Bus CV and gate

ID	Why
BUS-016	The bus CV and gate lines are shared across a row, so a module left at its default never fights a transmitter or reacts to stray levels.
BUS-017	Every listening oscillator follows the one bus CV line, so shared levels and impedances keep any transmitter and listener in tune.
BUS-018	Shared thresholds and impedances let any module's gate on the bus register reliably at every listening module.

6.7 12 V DC power (PWR-DC12)

ID	Why
DC-001	The variant tells a buyer which plug, cable, and distribution product will physically mate with the product.
DC-002	Nominal 12 V adapters vary in output, so the accepted range tells a buyer which adapters the product can safely use.
DC-003	A DC entry wired like a bus header invites a bus ribbon to be plugged into it, putting 12 V across the wrong pins.
DC-004	Center positive is the usual convention of 12 V adapters, so fixing it keeps a common adapter from reversing the DC rail.
DC-005	Fixed pin functions let any cable or distribution product of that variant deliver +V and −V to the right pins.
DC-006	Center-negative adapters fit the same barrel, so a reversed adapter is a likely mistake that must not destroy the module.
DC-007	A straight cable has to keep the center-positive polarity of both ends, or it reverses the DC rail it carries.
DC-008	Pin-for-pin wiring keeps +V and −V where the far end expects them, so the cable never reverses or shorts the DC rail.
DC-009	A DC-BAR25 plug enters a DC-BAR21 entry with unreliable contact, so outputs to modules use only connectors that fit module entries properly.
DC-010	Module entries are center positive with pin 1 at +V, so a distribution cable has to keep that polarity at both ends.
DC-011	An adapter is chosen at the inlet; the polarity symbol, the voltage, and the current beside it are what keeps a wrong adapter off a product.
DC-012	One shorted cable on an unprotected output takes every module on the rail down; a limit per output keeps the fault to one module.
DC-013	An entry that feeds a row is plugged by whoever assembles the case; a reversed brick must not take the row with it.

6.8 ±6 V analog power (PWR-A6)

ID	Why
A6-001	One connector family used by no other domain keeps a PWR-DC12 or bus cable from mating an analog entry; the pin functions make any PWR-A6 cable fit any entry.
A6-002	A module's local regulators are designed to this window; it holds the ±4 % of common fixed regulators.
A6-003	The 0.2 V below the outlet window covers the cable and connector drop at the entry limit, so any conforming source works with any conforming module.
A6-004	A source may overshoot by 10 % at start-up (SUP-006); the module must survive what a conforming source can deliver.
A6-005	The limit keeps the cable drop inside the 0.2 V allowance and lets a source rate its outlets once for every module.
A6-006	A straight-through cable keeps the pin functions; the length and gauge hold the drop at 1 A inside the window allowance.
A6-007	The A6-VH3 housing is keyed, and the marking tells an installer what the connector is before a cable is sought; a two-circuit VH housing fits two of its pins, so the ±6 V mark beside it is what stops a DC-VH2 cable going there.

6.9 ±15 V analog power (PWR-A15)

ID	Why
A15-001	One connector family used by no other domain keeps a PWR-DC12 Mini-Fit case entry (six or eight circuits) or any other cable from mating a ±15 V entry; the pin functions make any PWR-A15 cable fit any entry.
A15-002	A module's local regulators are designed to this window; it holds the ±4 % of common ±15 V supplies.
A15-003	The 0.2 V below the outlet window covers the cable and connector drop at the entry limit, so any conforming source works with any conforming module.
A15-004	A source may overshoot by 10 % at start-up (SUP-006); the module must survive what a conforming source can deliver.
A15-005	The limit keeps the cable drop inside the 0.2 V allowance and lets a source rate its outlets once for every module.
A15-006	A straight-through cable keeps the pin functions; the length and gauge hold the drop at 2 A inside the window allowance.
A15-007	The A15-MF4 housing is keyed, and the marking tells an installer what the connector is before a cable is sought; a two-circuit VH housing fits two of its pins, so the ±15 V mark beside it is what stops a DC-VH2 cable going there.

7.1 Signals

ID	Why
SIG-001	The port list tells a user which class and limits each jack meets, so a patch can be checked before it is made.
SIG-002	±12 V is the most any rail or conforming output can put on a jack, so an input that withstands it survives any patch.
SIG-003	Patching an output to 0 V or to another output is a routine mistake on a patch panel, and one cable must not destroy a module.
SIG-004	Keeping outputs within ±12 V keeps them within what every patch input is built to withstand under SIG-002.
SIG-015	The video reference is system configuration, not a patch: it stays on the rear coaxial interface, so that patching never changes a module's timing reference and no patch cable has to be a system cable.

7.2 Patch signal classes

ID	Why
SIG-005	Modules in use exceed the typical 10 V peak-to-peak audio level, and an input has to accept what an output may send.
SIG-006	Shared control limits let an envelope or LFO from one maker drive a control input from another without clipping.
SIG-007	One scale lets any pitch source drive any oscillator; the absolute pitch at 0 V is a recommendation, since every oscillator has a tune control.
SIG-008	Shared video limits let pictures patch between video modules from different makers at the right level.
SIG-009	A high, tightly held input impedance lets a 75 Ω output arrive at almost unity gain, so video levels match from module to module.
SIG-010	A 75 Ω series source keeps a short unterminated patch cable from ringing, and one fixed value keeps level and edges alike from module to module.
SIG-011	The Doepfer thresholds, high at +3 V and low at +1 V, let any gate output switch any gate input with margin against noise.
SIG-012	Shared thresholds and a minimum width make a short trigger long enough for any receiving input to catch it.
SIG-013	Clock outputs and inputs share the gate thresholds, so any clock source drives any clocked module reliably.
SIG-014	A named range is the contract between an output and an input, so matching ranges patch together without clipping.
SIG-016	Whether a path carries the format a buyer runs is the one thing they need to know before patching two video modules; a published figure, and the tier it meets, says it. A slow path, such as a filter, is a design, and the figure says that too.

7.3 Video timing

ID	Why
VID-001	Listing formats by ID on the reference ports lets a buyer check that a video reference generator and the modules that lock to it share a format; a patch jack lists none, because it is never the reference.
VID-002	Two products locked to one format have to agree on its raster; the publication's tolerances, not a clock-exact row, are what an analog interlaced format can be held to, while a sample-defined format is exact.

7.4 Video on 75 Ω coaxial jacks (VID-75)

ID	Why
V75-001	The three jack types are electrically the same here, so any 75 Ω cable with the matching plugs joins any two products.
V75-002	One center and shell convention lets any coaxial cable carry the signal and join the 0 V of the products it connects.
V75-003	The number stays unused; a port claimed by kind needs no second publication.
V75-004	The number stays unused; the termination rule of the port mode covers the designated input.
V75-005	A second termination on one output drops the level to two thirds and a third drops it to half.
V75-006	The number stays unused; the rule was conditional on how an installer cabled a port, which a product cannot know.
V75-007	A 75 Ω cable keeps the line impedance continuous from source to termination, so the star stays matched and free of reflections.
V75-008	The number stays unused.
V75-009	The number stays unused; a feature describes what happens behind the panel or inside the case, and a front picture jack is an external connection like line-level audio.
V75-010	The reference is system cabling: at the rear, on the coaxial interface, it is wired once behind the panel and stays out of the patch field.
V75-011	VID-REF-IN says what a buyer needs to know first: this module follows the row's reference, through this rear port, in these formats.
V75-012	An output claim means the module can be the row's reference; a pass-through cannot, and the two are easy to confuse on a panel marked OUT.
V75-013	A buffered pass-through lets a reference chain through modules without a distribution product, and the mark keeps it from being cabled as a source.
V75-014	A loop-through chains a reference through several modules on one cable run with one termination at the end; the switch is what lets the last module terminate it.
V75-015	The reference tree works only when every port in it is terminated once and driven once; the port modes say what each jack does. A picture port on the front is an external interface in the port list and claims nothing.
V75-016	The installer cables the reference once, behind the panel, and reads the marking, not the manual.
V75-017	A tri-level video sync edge is 54 ns ± 20 ns (SMPTE ST 240); a 30 MHz path keeps it inside that tolerance through a chain of pass-throughs, and the buffers the trade uses for reference distribution clear it with margin.

8.1 USB service port and parameter interface (CTRL-USB)

ID	Why
USB-001	One connector and pinout behind the panel lets one short straight lead join any CTRL-USB product to any case host.
USB-002	The housing is common: a lead made for another purpose can be fitted to the port, and the marking and the withstand keep that from damaging the product.
USB-003	In a row the product runs from its power entries; a host can then budget every port at 100 mA, and no product back-feeds the host's 5 V.
USB-004	A straight lead keeps the pinout; the length and the twisted pair keep full-speed signalling inside a case.
USB-005	A host names a product from iProduct and tells two units apart by iSerial, including after a move to another port or a reboot into a boot loader.
USB-006	MIDI-CI identifies a device by its manufacturer ID; 7D is reserved for non-commercial and development use and would collide between makers. A manufacturer ID is a paid, one-time registration with the MIDI Association, the one cost CTRL-USB puts on a maker.
USB-007	The USB-IF MIDI 2.0 class is what hosts and computers already drive; one endpoint pair keeps a host's endpoint budget small, and alternate setting 0 serves MIDI 1.0 hosts.
USB-008	These standard resources are all a host needs to list a product's controls and name them, with no product-specific code.
USB-009	Without the map or the step count a generic host can only show a percentage; 32-bit controller messages carry the full resolution of the entry's range.
USB-010	MIDI 2.0 has no read-back of a controller's value; this convention lets a host show current values with standard messages only.
USB-011	A host with few endpoints parks the devices it is not showing; a device that stops when parked would be lost to the host.
USB-012	Serving the schema is how a host knows the x-units it reads mean what EIS says; a host ignores x-units from a device that does not serve it.
USB-013	One stuck or shorted product must not take the other ports with it, and the host can restart one product without disturbing the rest.
USB-014	The user finds the product the host shows by the port it is plugged into.
USB-015	Any conforming product then works with any conforming host, including products the host's maker never saw.
USB-016	Values sent while the endpoints were closed are lost; a new subscription brings them back (USB-010).
USB-017	One protocol lets any computer software reach products behind any host, for their consoles and their firmware updates.

9 Assembling a system

ID	Why
ASM-001	Exceeding a bus board, header, or supply rating overheats contacts or trips the supply; the published currents exist so it can be checked first.
ASM-002	Two transmitters on one line drive against each other, leaving a level that is neither and stressing both outputs.
ASM-003	Two sources on one module can feed current from one into the other through the module's power circuit.
ASM-004	A DC-BAR25 plug enters a DC-BAR21 entry but makes unreliable contact, which can run hot or drop power under load.
ASM-005	Two adapters or entries on one DC rail fight each other's regulation and can feed current back into each other.
ASM-006	Each multi-pin variant has its own pin functions and contact rating, so only its own plug puts +V and −V on the right pins at its rated current.
ASM-007	Two sources on one tree fight; one input on two outputs is a passive split.
ASM-008	Two sources on one entry fight each other's regulation and can back-feed one another.
ASM-009	A supply and a bus board from different makers are joined by the installer; the wiring rules travel with the installer, not with either product.