finish mechanical specifications

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diff --git a/mechanical-specifications.typ b/mechanical-specifications.typ
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--- a/mechanical-specifications.typ
+++ b/mechanical-specifications.typ
@@ -886,3 +886,388 @@ least $qty(4, "mm")$ height. Jack labels should be at least $qty(2, "mm")$
 height.
 
 == Chassis <chassis>
+
+=== Overview <chassis-overview>
+
+The chassis provides the mechanical and electrical infrastructure for housing
+modules. It includes:
+- Structural enclosure with EMI shielding,
+- Cassette rail system for module installation and removal,
+- Power distribution board with backplane connectors,
+- Front panel with reference outputs and system controls,
+- Thermal management provisions.
+
+This specification defines interface requirements that any compliant chassis
+must meet. Chassis may vary in capacity (number of module positions), form
+factor (rack-mount, desktop, portable), and mounting style. The reference
+implementation (#ref(<reference-implementation-chassis>)) describes a
+$qty(19, "inch")$ $qty(4, "U")$ rack-mount chassis with $8$ module positions.
+
+=== Dimensional requirements <chassis-dimensional-requirements>
+
+#figure(
+  image("pictures/chassis-module-loading.svg"),
+  caption: [Chassis module loading dimensional requirements],
+) <figure-chassis-dimensional-requirements>
+
+==== Module interface dimensions <chassis-module-interface-dimensions>
+
+All chassis must accommodate standard SAME cassettes. The following dimensions
+are mandatory:
+
+#figure(
+  table(
+    columns: (auto, auto, auto),
+    table.header([Parameter], [Value], [Notes]),
+    [Module pitch], [$qty(38.1, "mm")$ ($qty(1.5, "inch")$)], [Center-to-center spacing],
+
+    [Module opening height], [$qty(177.8, "mm")$ ($qty(4, "U"), qty(7, "inch")$)], [Panel visible area],
+
+    [Module body clearance], [$qty(165.2, "mm")$], [Between top and bottom rails],
+
+    [Module depth], [$qty(152.5, "mm")$], [Front panel to rear connector],
+    [Rail-to-rail width], [$qty(38.1, "mm")$ ($qty(1.5, "inch")$)], [Center-to-center spacing],
+  ),
+  caption: [Module interface dimensions],
+) <table-chassis-module-interface-dimensions>
+
+==== Capacity <chassis-capacity>
+
+Chassis capacity is defined by the number of module positions. There is no
+minimum or maximum, but the following constraints apply:
+
+#figure(
+  table(
+    columns: (auto, auto, auto),
+    table.header([Capacity], [Power budget], [Notes]),
+    [Per module], [$qty(185, "mA") at plus.minus qty(15, "V")$], [Maximum per position],
+
+    [Per module], [$qty(100, "mA") at plus.minus qty(5, "VD")$], [Maximum per position],
+
+    [Total], [Scales with capacity], [Power distribution must support all positions],
+  ),
+  caption: [Chassis capacity power requirements],
+) <table-chassis-capacity>
+
+Designers should consider thermal dissipation when determining maximum capacity.
+
+==== Clearances
+
+The rear clearance (behind cassette) should be at least $qty(25, "mm")$, to
+account for the DB-25 connector and cable bend radius.
+
+=== Structural requirements <chassis-structural-requirements>
+
+==== Material <chassis-material>
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Parameter], [Requirement]),
+    [Material], [Steel (recommended) or aluminum],
+    [Minimum thickness], [$qty(1.5, "mm")$ (steel) or $qty(2.0, "mm")$ (aluminum)],
+
+    [Finish], [Black powder coat or anodization, conductive areas masked],
+  ),
+  caption: [Chassis material requirements],
+) <table-chassis-material>
+
+Steel is preferred for magnetic shielding. Aluminum may be used where weight is
+critical, with reduced magnetic shielding performance.
+
+==== EMI shielding <chassis-emi-shielding>
+
+A fully loaded chassis must form a continuous shielded enclosure:
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Parameter], [Requirement]),
+    [Shielding effectiveness], [$gt qty(20, "dB"), qtyrange(80, 1000, "MHz")$],
+    [Panel joints], [Metal-to-metal contact of conductive gasket],
+    [Ventilation openings], [\ Maximum slot dimensions: $qty(10, "mm") times qty(2, "mm")$],
+
+    [Cable entry], [Shield connectors or filtered feedthroughs],
+  ),
+  caption: [Chassis EMI shielding requirements],
+) <table-chassis-emi-shielding>
+
+==== Grounding <chassis-grounding>
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Parameter], [Requirement]),
+    [Chassis earth bonding], [All panels bonded to single earth point],
+    [Earth impedance], [$gt qty(0.1, "ohm")$ between any two chassis points],
+    [Safety earth], [Connected to mains earth via IEC inlet],
+    [Module earth path], [Via DB-25 shell and pin 25],
+  ),
+  caption: [Chassis grounding requirements],
+) <table-chassis-grounding>
+
+=== Cassette rails <chassis-cassette-rails>
+
+The rail system guides cassette insertion and provides retention.
+
+==== Geometry <chassis-cassette-rails-geometry>
+
+#figure(
+  table(
+    columns: (auto, auto, auto),
+    table.header([Parameter], [Value], [Notes]),
+    [Rail height], [$qty(6.35, "mm")$ ($qty(0.25, "inch")$)], [Matches cassette lip height],
+
+    [Channel width],
+    [$qty(38.1, "mm")$ ($qty(1.5, "inch")$) $plus 0.2 "/" minus qty(0.0, "mm")$],
+    [Accepts cassette body with clearance],
+
+    [Channel depth], [$qty(5, "mm")$ minimum], [Guides cassette during insertion],
+
+    [Rail length], [Full chassis depth], [Front to rear],
+  ),
+  caption: [Chassis cassette rails geometry],
+) <table-chassis-cassette-rails-geometry>
+
+==== Retention <chassis-cassette-rails-retention>
+
+Cassettes must be retained to prevent accidental ejection during operation or
+transport.
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Method], [Requirement]),
+    [Friction fit], [Acceptable for stationary installations],
+    [Positive retention], [Required for portable or vibration-prone environments],
+
+    [Front panel screws], [Optional, into chassis front rail],
+  ),
+  caption: [Chassis cassette retention requirements],
+) <table-chassis-cassette-rails-retention>
+
+Positive retention may be achieved via:
+- Spring clips engaging cassette lips,
+- Latching mechanisms.
+
+==== Material <chassis-cassette-rails-material>
+
+The rails must be of the same metal as the chassis, with a smooth, burr-free
+finish; and bond to chassis earth.
+
+They contribute to EMI shielding by closing the gap between cassette lips and
+chassis structure.
+
+=== Thermal management <chassis-thermal-management>
+
+==== Cooling method <chassis-cooling-method>
+
+Passive convection cooling is the baseline requirement. Forced-air cooling may
+be added for high-density configurations.
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Method], [Requirement]),
+    [Passive (convection)], [Minimum for all chassis],
+    [Active (forced air)], [Optional, for high-capacity or enclosed installations],
+  ),
+  caption: [Chassis cooling method requirements],
+) <table-chassis-cooling-method>
+
+==== Ventilation <chassis-ventilation>
+
+Chassis ventilation must align with cassette ventilation slots to enable
+chimney-effect airflow.
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Parameter], [Requirement]),
+    [Bottom ventilation], [Required, aligned with cassette bottom vents],
+    [Top ventilation], [Required, aligned with cassette top vents],
+    [Slot dimensions], [$lt.eq qty(10, "mm") times qty(2, "mm")$ for EMI compliance],
+
+    [Open area], [$gt.eq qty(20, "percent")$ of cassette footprint area],
+  ),
+  caption: [Chassis ventilation requirements],
+) <table-chassis-ventilation>
+
+==== Thermal limits <chassis-thermal-limits>
+
+Inside the chassis cassette chamber, internal ambient temperature must not go
+over $qty(50, "dC")$. If passive cooling cannot maintain thermal limits at full
+capacity, forced-air cooling or reduced capacity is required.
+
+=== Power distribution board <chassis-power-distribution-board>
+
+The power distribution board generates and distributes all power rails to
+module positions.
+
+==== Functions <power-distribution-board-functions>
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Function], [Requirement]),
+    [AC-DC conversion],
+    [Mains input to $plus.minus qty(15, "V")$, $plus.minus qty(5, "VD")$ rails. Can be external or internal],
+
+    [Reference generation], [$plus.minus qty(10.0000, "V")$ precision references],
+
+    [Master oscillator generation], [Generation of the $qty(10, "MHz")$ sine wave reference],
+
+    [Distribution],
+    [
+      - One female DB-25 connector per module position,
+      - One female isolated BNC connector per module position
+    ],
+
+    [Calibration], [Front-panel accessible switches],
+    [Protection], [Overcurrent, overvoltage, thermal shutdown],
+  ),
+  caption: [Power distribution board functional requirements],
+) <table-power-distribution-board-functions>
+
+==== Power entry <power-distribution-board-power-entry>
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Parameter], [Requirement]),
+    [Mains connector], [IEC C14 (with integrated fuse) or regional equivalent],
+    [Voltage range],
+    [$qtyrange(100, 240, "VAC"), 50 "/" qty(60, "Hz")$ (universal) recommended. Factory setup accepted],
+
+    [Safety], [Compliant with IEC 60950-1 or IEC 62368-1],
+  ),
+  caption: [Power entry requirements],
+) <table-power-distribution-board-power-entry>
+
+==== Master oscillator <power-distribution-board-master-oscillator>
+
+To support the metrologic tier time-domain computations, the chassis must
+include a precision clock source.
+
+/ Oscillator: Oven Controlled Crystal Oscillator (OCXO).
+  / Recommended part: AOCJY4A-10.0000MHZ-SW (or equivalent with
+    $plus.minus qty(10, "ppb")$ stability).
+/ Buffering: The oscillator output must be filtered to remove harmonics and
+  buffered to drive the $qty(50, "ohm")$ distribution network.
+  / Metrologic tier buffer: LM7171 (high speed, high voltage operational
+    amplifier) or equivalent, configured to drive $qty(50, "ohm")$ loads with
+    high slew rate and low distortion.
+/ Signal level: $qty(1.0, "Vpp") plus.minus qty(10, "percent")$ into
+  $qty(50, "ohm")$.
+/ Waveform: Sine wave.
+/ Fan-out: The oscillator distribution network must ensure port-to-port
+  isolation $gt qty(40, "dB")$ to prevent oscillator line crosstalk between
+  modules.
+
+=== Front panel <chassis-front-panel>
+
+The chassis front panel provides system-level controls and reference inputs and
+outputs.
+
+==== Elements <chassis-front-panel-elements>
+
+#figure(
+  table(
+    columns: (auto, auto, auto),
+    table.header([Element], [Type], [Function]),
+    [SAME logo], [Label], [N/A],
+    [SAME license], [Label], [Legal],
+    [Power switch], [Rocker or toggle], [Mains on/off],
+    [Standby switch], [Toggle], [Maintains reference ovens temperature],
+    [Power indicator], [LED], [Indicates power state],
+    [Standby indicator], [LED], [Indicates standby state],
+    [$plus qty(10.0000, "V")$ output jack], [4mm banana, yellow], [Reference voltage output],
+
+    [$qty(-10.0000, "V")$ output jack], [4mm banana, blue], [Reference voltage output],
+
+    [$plus qty(10.0000, "V")$ input jack], [4mm banana, white], [Reference voltage input],
+
+    [$qty(-10.0000, "V")$ input jack], [4mm banana, white], [Reference voltage input],
+
+    [AGND jack], [4mm banana, black], [Analog ground reference],
+    [Dual $qty(10, "MHz")$ sine oscillator output jacks $plus.minus qty(1, "V")$],
+    [4mm banana, light blue],
+    [Master oscillator output (independent paths, to allow verification - see #ref(<the-time-invariant>))],
+
+    [$qty(10, "MHz")$ sine oscillator input jack], [4mm banana, white], [Master oscillator input],
+
+    [Calibration coarse], [Rotary or DIP switch], [Reference voltage coarse adjust],
+
+    [Calibration fine], [Rotary or DIP switch], [Reference voltage fine adjust],
+  ),
+  caption: [Chassis front panel elements],
+) <table-chassis-front-panel-elements>
+
+- Case designers may add other elements as they see fit (fuse holders, voltage
+  selectors, sensors, etc.),
+- Reference jacks should be grouped together, on a $qty(0.75, "inch")$ grid,
+- All controls and jacks must have an associated label.
+
+=== Electrical safety <chassis-electrical-safety>
+
+==== Mains isolation <chassis-mains-isolation>
+
+#figure(
+  table(
+    columns: (auto, auto),
+    table.header([Parameter], [Requirement]),
+    [Isolation voltage], [$gt.eq qty(3000, "VAC")$ mains to SELV],
+    [Creepage / Clearance], [Per IEC 60950-1 or IEC 62368-1],
+    [Fusing], [Primary slide, appropriately rated],
+    [Earth bonding], [Continuous, $lt qty(0.1, "ohm")$ to mains earth],
+  ),
+  caption: [Mains isolation requirements],
+) <table-chassis-mains-isolation>
+
+==== SELV circuits <selv-circuits>
+
+All module-facing voltages ($plus.minus qty(15, "V")$, $plus.minus qty(10, "V")$, $plus.minus qty(5, "V")$) are Safety Extra-Low Voltage (SELV):
+
+#figure(
+  table(
+    columns: 2,
+    table.header([Parameter], [Requirement]),
+    [Maximum voltage], [$lt qty(60, "VDC")$],
+    [Isolation from mains], [Required],
+    [User accessible], [Yes (banana jacks)],
+  ),
+  caption: [SELV circuits requirements],
+) <table-selv-circuits>
+
+==== Protective earth <protective-earth>
+
+#figure(
+  table(
+    columns: 2,
+    table.header([Parameter], [Requirement]),
+    [Earth continuity], [$lt qty(0.1, "ohm")$ from any exposed metal to mains earth],
+
+    [Earth current capacity], [Per fuse rating],
+    [Chassis bonding], [All panels bond],
+  ),
+  caption: [Protective earth requirements],
+) <table-protective-earth>
+
+=== Compliance <chassis-compliance>
+
+Chassis must comply with applicable safety and EMS standards for the target
+market:
+
+#figure(
+  table(
+    columns: 3,
+    table.header([Region], [Safety], [EMC]),
+    [EU], [IEC 62368-1], [EN 55032, EN 55035],
+    [USA], [UL 62368-1], [FCC Part 15],
+    [International], [IEC 62368-1], [CISPR 32, CISPR 35],
+  ),
+  caption: [Chassis compliance requirements],
+) <table-chassis-compliance>
+
+Compliance testing and certification are the responsibility of the chassis
+manufacturer.