doc: Document the enablement circuit mod

PCB.md

@@ -2,4 +2,22 @@
 - The DB9 male port (CAN1) is directly connected to the DB9 female port (CAN2) on all but pins 1, 5 and 8.
 
 # Operating the board
-- 7V on pin 8, and GND on pin 3, is enough to operate the board, per my Signal conversation with Tim on 10 Feb 2024 @ 4:22AM
+
+## Pinout
+
+### CAN
+- 2: CAN_L
+- 3: CAN_GND (low-impedance to pin 6, GND)
+- 7: CAN_H
+
+### Power
+- 4: V_BAT
+- 6: GND
+- 8: ENABLE
+
+## Enablement
+The OEM enabement mechanism is not known.
+
+A small modification can be made to allow ENABLE to be (in effect) a default pulled-down, on-if-high pin for ~2.8V V_ON.
+
+Place a 800-1.2k resistor between the existing ENABLE pin line (for example, on one of the pads near the bottom right corner of the board) and pin 2 of the 752T SOIC-8. This pin 2@752T connects fairly directly into the center of the enable logic circuitry (it is the ON pin of the 752T, as well as one of the inhibitor pins keeping the LM25010 off).