Seriss 1A2 Single Board KSU - Board Revisions
(Revisions shown newest first, oldest last)

"REV J1" (Jan 2020) Differs from "REV J" in the following ways:
  * Modification for Northern Telecom LOGIC 10; swapped buzzer leads [1]
  * Brought EXT 1-4 connectors closer by a small bit so phones w/3 amphenols
    don't stretch when all connected
  * Small trace mods, no electrical changes (other than LOGIC 10 mod above)
  * Enlarged many solder pads to aid in soldering (chips, discrete components..)
  * Enlarged clearance for ground trace away from amphenol signal bus and JP3 T1 via
  * Small trace changes to remove some unnecessary vias
  * ULN2803 (IC12) and 10K RNET (RN2) lowered to clear for "BUZZERS"
    label and to remove an unnecessary via
  * Simplified trace runs around "12V INPUT"
  * Dropped 3rd pad hole on left of R28, will never be needed.
  * Labeling changes:
      - Added "CPU1" and "CPU2" labels to PIC chips.
      - Added part# for 220 ohm resistors
      - Flopped "POWERDSINE RING GEN" label for JP5 to match others
      - Many section labels enlarged
      - Removed "A" and "B" labeling on PRIMARY/SECONDARY connectors
        because they weren't being used.

  [1] This change prevents board from immediately shorting out when a LOGIC 10
      is trivially plugged in without wiring reassignments. Flopping buzzer
      polarity so ground is on 17 instead of 42 to solve. LOGIC 10 makes odd use
      of Y-O pair for A lead ground.

"REV J" (Dec 2019) Differs from "REV H2" in the following ways:
  * Polarity swapped for A lead circuits to support Nothern Telecom LOGIC6 and LOGIC10
  * Fixed missing ground for L4 ground on EXT2 (this was in H1 and H2, fixed in H3)
  * Increase clearance of JP4's "SEC" trace across board to be away from TO220 screws
  * Increases size of annular rings -- printers had trouble with the solder mask
  * Enlarged layout for capacitors slightly
  * Increased DRC (Design Rule Check):
    * Copper 5.9mil -> 7.0mil
    * Annular ring min: 5.0mil -> 7.0mil

"REV H2" (Nov 2019) Differs from "REV H1" in the following ways:
  * Fixed badly positioned via at end of EXT1's T1 trace, off by a full grid unit.
  * Removed part#s for non-existant JP1/2 at far right of board
  * Moved JP3 part# from board edge to the component's location
  * Removed redundant part# for JP3/4 at far right of board; components have this info already.
  * Removed dated PCB: and SCH: filenames from right side of board: never bothered to update them!
  * Enforce DRC (Design Rule Check):

      Minimum Distance (mil)
          (x) Copper: 5.9
          (x) Drillings: 19.7
      Min/Max: (mil)
          (x) Drilling Min/Max: 11.8/315.0
          (x) Track Min: 6.0
          (x) Annular Ring Min: 5.0
          (x) Silkscreen Min: 5.0
      Miscellaneous: (these are all unchecked)
          ( ) Silkscreen over pads
          ( ) Drilling on SMD pads
          ( ) Pads without soldermask
          ( ) Soldermask out of pads

"REV H1" (Oct 2019) Differs from "REV H" in the following ways:

    * Extra holes for 160 OHM HOLD RESISTOR (R1+R8)
      Copied one of the RING DETECT resistors, which checked + worked.

    * Changed Digikey part# for 33K 1/2W to be 1W component

    * Changed Digikey part# for 0.1uF caps to be bulk (instead of cut tape)

    * Changed 0.1uF caps' footprints to be smaller, so leads can be
      folded onto the component (instead of needing to leave 1/4" leads)

    * Changed holes for TIP125 and Amphenols to be plated through holes
      (like the TIP32's) to prevent PCB printing delays (happened on last order).

    * Fixed small horiz alignment problem with EXT1

    * Added pin1 "dot" to all IC sockets

    * Standardized IC sockets pad/holes to 56mil/29mil (some were 55.x)

    * Removed D4-D11 (replaced with IC13, see below)

    * Added IC13: converted 8 1n914 BUZZ CALL diode arrangement (D4-D11)
      to a single 20 PIN IC socket for the 10 LED BARGRAPH. This to make
      it easy to simply insert the bargraph component into the board
      without having to cut off the lower 2 rows of pins. Doing this
      necessitated rerouting some of the other traces to make room for
      the 2 new rows of pads for the unused LEDs. And populating this position
      on the board has ALWAYS been done with the LED bargraph, not 8 discrete
      1n914s (which also works, but harder to solder)

    * Updated all part#s in component comments to part#s that are for "bulk" parts
      (instead of cut tape)

    --- REV-H1:
    #   o Extra holes for HOLD RESISTOR
    #     Basicalled moved one of the RING DETECT resistors up.
    #   o Changed digikey part# for 60 / 1W res from cut-tape to bulk
    #     RSF1JT62R0CT-ND -> 62W-1-ND
    #         \                  \
    #          \__ cut tape       \__ bulk
    #              (old)              (new)
    #   o Changed digikey part# for 1K / 0.25W res from cut-tape to bulk
    #     CF14JT1K00CT-ND -> 1.0KQBK-ND 
    #         \                  \
    #          \__ cut tape       \__ bulk
    #              (old)              (new)
    #   o Changed digikey part# for 1K / 1W res from cut-tape to bulk
    #     RSF1JT1K00CT-ND -> 1.0KW-1-ND
    #         \                  \
    #          \__ cut tape       \__ bulk
    #              (old)              (new)
    #   o Changed digikey part# for 10K / .25W res from cut-tape to bulk
    #     CF14JT10K0CT-ND -> 10KQBK-ND
    #         \                  \
    #          \__ cut tape       \__ bulk
    #              (old)              (new)
    #   o Changed digikey part# for 100K / .25W res from cut-tape to bulk
    #     CF14JT100KCT-ND  -> 100KQBK-ND
    #         \                 \
    #          \__ cut tape      \__ bulk
    #              (old)             (new)
    #   o Changed digikey part# for 33K to be 1W component 33KWCT-ND -> 33KW-1-ND
    #     These are the same size as the 1K/1W's.
    #   o Changed digikey part# for 70K / .25W res from blue 5-ring to brown 4-ring
    #     69.8KXBK-ND   ->  68KQBK-ND
    #         \                 \
    #          \__ blue/5ring    \__ brown/4ring
    #              (old)             (new)
    #   o Changed digikey part# for 150K / .25W res from cut-tape to bulk
    #      CF14JT150KCT-ND -> 150KQBK-ND
    #         \                 \
    #          \__ cut tape      \__ bulk
    #              (old)             (new)
    #   o Changed digikey part# for 0.1uF caps to be bulk (instead of cut tape)
    #     399-4454-1-ND -> 399-9843-ND
    #         \                 \
    #          \__ cut tape      \__ bulk
    #              (old)             (new)
    #   o Changed footprint for all 0.1uF caps to be smaller, so leads can be
    #     folded onto the component, instead of leaving extra little 1/4" leads
    #   o Changed holes for TIP125 + Amphenol's to be plated through holes (like the TIP32's)
    #     to prevent PCB printing delays that happened on last order.
    #   o Added part# label to fourh 1n4148 diodes

"REV H" (Sep 2019) Differs from "REV G1" in the following ways:
    * New input SECONDARY_DET on Cpu1/RC3 (currently unused)
    * New input SECONDARY_DET on Cpu2/RA5 (currently unused)
    * New onboard fuse
    * Removed JP1 and JP2 "PIC PROG" connectors, obsolete
    * Trace clearances increased around some interlink pads
    * Small printing error:
      In board run "WELLPCB_REV-H-printed-09-16-2019", bottom silk revision marker
      said "G1" instead of "H". Fixed lay6 file + gerbers for next board run,
      (but left WELLPCB directory unchanged)

"REV G1" (Jul 2019) Differs from "REV G" in the following ways:
    * No impact on software at all
    * Added TIP125 surge suppression diodes D22-D25
    * Change surface mount resistors to through hole (SMT annoying to solder)
    * Repositioned R25, R26, and R28 (changed to R27) for easier soldering/lead trimming
    * Swapped component numbers for R28 <-> R27: regional sequential numbering
    * Moved C7 next to C5 + C6 for easier assembly
    * Moved JP5 to left of EXT1 for cable clearence
    * Increased INTERLINK connector pad size for *solder side only* (more surface area for soldering)
    * Increased overall pad size for JP3 & 4 (more surface area for soldering)
    * Slight enlarge of LED holes to allow red status leds to flush mount
    * Added extra pad holes for 60/1W and 300/1W resistors

"REV G" (Jun 2019) Differs from "REV F" in the following ways:
    * REV G added 'SYNC' I/O signal to allow two boards to keep their
      flashing signals in sync (Hold + Ring Flash). Mostly an INPUT,
      briefly switches to output to send short sync signal to "other" cpu.
      This ensures "slower" cpu autosyncs itself to "faster" one.
      See UpdateInterlinkSync() function for complete implementation.
    * LED through holes enlarged slightly allowing red CPU LEDs to flush mount
    * Some traces moved around for clearance issues, but no electrical changes

"REV F" and "REV F1" (May 2019) Introduced interlink feature, two cpus.
    Differs from "REV E" in the following ways:
    * Revision "F" was an internal "development only" board; "F1" is the release version.
    * Introduiced Interlink feature to support combining two boards for 4 lines/8 extensions
    * Introduced rotary dial support for extension dialing.
    * Now has two PIC chips; CPU1 and CPU2, each with separate firmware
    * CPU1 handles logic same as REV E's single CPU, but no longer handles MT8870 signals
    * CPU2 handles MT8870 signals and rotary dialing
    * HandleDTMF() function removed from CPU1 firmware, moved to CPU2
    * Dropped these components:
           * "REV E" JP2 removed  -- interlink handles this now
           * "REV E" C4 removed   -- 2200uF filter capacitor (voice batt) was unused
           * "REV E" 7445 removed -- made obsolete by use of PIC CPU2

"REV E" (Apr 2019) First version of board with PIC chip to handle logic.
    * Previous board used no CPUs, all analog/discrete circuitry.
    * This board included 8 buzzer transistors and no interlink feature.