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|'''J-16'''||In/Out||Ribbon to embedded controller — DAC code in, telemetry out via optocouplers
|}
=== J-12 Power-Input Topology ===
J-12 carries three transformer secondary windings plus a single ground-tie wire. The transformer itself has no center tap or earthed reference — the system ground is '''established on the board''' by bonding the negative DC output of one rectifier to the positive DC output of another, using the lone green wire. This stacks two of the rectified supplies end-to-end to produce the bipolar ±V<sub>unreg</sub> rails feeding the ±15 V analog regulators, while the third (blue) winding feeds an independent positive rail for the digital regulators.
==== Stacked Topology ====
<pre>
+Vunreg ──▶ μA 78M15 ──▶ +15 V analog
▲
┌────────┴────────┐
│ Yellow pair │ ← secondary winding #1
│ → bridge rect. │
└────────┬────────┘
│ "−" output of yellow rectifier
●━━━━━━━━━━━● ◀── GREEN wire (system ground, 0 V)
│ "+" output of red rectifier
┌────────┴────────┐
│ Red pair │ ← secondary winding #2
│ → bridge rect. │
└────────┬────────┘
▼
−Vunreg ──▶ μA79M15A ──▶ −15 V analog
┌─────────────────┐
│ Blue pair │ ← secondary winding #3 (independent)
│ → bridge rect. │
└────────┬────────┘
▼
+Vunreg(logic) ──┬──▶ LM340T-12 ──▶ +12 V
└──▶ μA 78M05 ──▶ +5 V digital
</pre>
==== Wire Color Map ====
{| class="wikitable"
! Wire !! Conductors !! Function !! Feeds
|-
| '''Yellow''' || pair || Secondary winding #1, feeds bridge rectifier whose "+" output becomes +V<sub>unreg</sub> || +15 V regulator (REG2)
|-
| '''Red''' || pair || Secondary winding #2, feeds bridge rectifier whose "−" output becomes −V<sub>unreg</sub> || −15 V regulator (REG1)
|-
| '''Green''' || single || Ground bond — ties "−" of the yellow rectifier to "+" of the red rectifier, establishing the 0 V system reference || Analog ground for the entire board
|-
| '''Blue''' || pair || Secondary winding #3 (independent), feeds bridge rectifier for the positive logic rail || +12 V (REG3) and +5 V (REG4) regulators
|}
==== Notes on the Topology ====
* The transformer has '''three independent floating secondaries''' — no center tap is brought out. The bipolar ±15 V rail pair is synthesized on the board by stacking two single-ended supplies via the green ground-tie wire.
* The green wire is a '''current-carrying ground return''', not just a reference. The imbalance current between the +15 V and −15 V loads flows through it back to the rectifier diodes, so it should be a reasonable gauge and routed for low loop inductance. Lifting it during service work will collapse the entire analog ground reference of the board.
* Because the "+" pin of the red bridge rectifier is bonded to ground, the red supply's '''negative''' DC output is the rail that goes down to the −15 V regulator. This is normal for a stacked topology but can be confusing if you expect the rectifier "+" pin to be the rail output.
* Keeping the digital +5 V / +12 V supply on its own winding (blue) isolates digital switching noise from the ±15 V analog rails, which carry the precision references for the AD7541 DAC and the AD654 V/F converters.
===Active Components===
