1A2 Multiline Phone Control Board - REV-J4

This revision adds two small improvements to the previous boards:

Synchronized ringing between interlinked boards
Improved lamp brightness for Intercom (Line 5) for long wire runs (100 ft or more)

[Update 11/01/2021] Received the REV-J4 boards from the printer, and soldered one up:

Fully assembled REV-J4 board

Raw REV-J4 boards from PCB printer

Features

This revision offers the same features as previous releases:

2 lines / 4 Extensions with intercom on Line 5
Expandable to 4 lines / 8 extensions if two boards are "interlinked" with a ribbon cable.
Intercom supports DTMF and Rotary dialing for buzzing extensions
All the standard 1A2 features for lamp flashing: Incoming call, call in progress, hold
Call transfer via Hold and Intercom
Multiline Bell ringing (via optional external Ring Generator)
Multiline Buzzer ringing (via built-in buzzer driver)
Programmable ringing for 2 lines 4 extensions via 8 position DIP switch
Disconnect Supervision releases calls on Hold when remote hangs up
Slots for amphenol velcro retaining straps (added REV-J3 and up)

If you are interested in purchasing assembled and tested boards, or have questions for more information,
contact erco@seriss.com, and include "1A2 KSU inquiry" in the email Subject.

When combining two boards to provide a total of 4 CO lines / 8 extensions, each extension can be uniquely dialed on the intercom line via Touch-Tone or Rotary. This page shows how to interlink two boards.

Closeup of cable management for interlinked boards

Slots between amphenols (new in REV-J3 and up) allow Velcro cable straps
(optional) to be added to retain connectors for permanent installations.

Eight 1A2 phones Interlinked with two KSU boards for a total of 5 active lines (4 CO and 1 Intercom) with two PowerDSINE Ring Generators.

Introduction

Many of these key phones are available in surplus (e.g. on Ebay), but can't be used as intended without a control box to manage features like Hold, lamps, intercom, etc. At the time miniaturization started becoming popular, the phone industry took a sharp turn from old electromechanical 1A2 equipment to smaller, more digital systems such as ComKey 416, Horizon, and by the late 80's, Merlin systems and the like which were pure digital. So the old 1A2 phone systems quickly became yesterday's technology by the 1980's.

My project with these circuit boards kinda picks up where the industry left off, to give those old 1A2 key phones a few more generations of use; it "miniaturizes" the functions of the KSU control into a single circuit board, making it easier to quickly create a small phone system of up to 4 of these phones, such as for use in small office environment, or to demo the old phones easily. 8 phones can be supported by interlinking two of these boards.

Here's a short YouTube video showing the card in action controlling four old 1A2 business phones.
It shows an early version of the card that used no CPUs and all discrete digital/analog components. In subsequent revisions, the design was changed to use one chip CPUs (PIC chips) to make it easy to add features, simplify the board layout, and decreasing overall cost.

I've switched my own business phone system over to one of these boards with 2 lines and 3 extensions, adding a Viking AR-1 to act as the front end autoattendant (blocks robocalls) and an answering machine to take calls when no one answers.

This 1A2 "single board KSU" project has evolved over time, resulting in a series of board revisions that accumulated improvements and features as it went. The project started with REV-B, and is now up to REV-J.

History of Board Revisions

complete revision history

Click here for a list of recent changes, and animations comparing revisions.

REV-J4

Support for V1.4 firmware was added. (Also supports V1.3 firmware)
This adds support for synchronized ringing, with "interrupter" behaviors more consistent with the original Western Electric KSU's: multiple incoming calls will all ring with matching ring cadence even across interlinked boards.
Basically V1.4 uses the same 'interrupter' model the old 1A2 KSUs use, where one "interrupter" drives all signals on both boards. In this case, the interrupter (implemented in firmware) on the PRIMARY board's CPU1 manages ring cadence, ring flash, hold wink signals used by both boards.
Previous revision boards (REV-J3 and older) use V1.3x, and won't work with the V1.4 firmware unless modified by cutting a single trace between CPU2's pin #2 and the nearby via. This is because V1.4 is the first firmware version to make use of the SECONDARY_DET signal, and when CPU2 is off/ICM hung up, the deenergized PIC's RC4 (pin #2) forces this signal low, which is bad. So cutting the trace prevents this problem, allowing the signal to be properly detected by CPU1.

The intercom circuit was modified to improve lamp brightness for Line #5 over long cable runs. In the earlier REV-J versions, the A lead circuit directly drove the intercom lamp, which meant the circuit took two round-trips down the 25 pair cable to light the Line #5 lamp, making it 1/2 as bright as the other lines. A relay was added to shorten this to a single round trip, making lamp brightness consistent with the other lines.

Small silk screen fixes; "LINE RINGING" section label was overlapping the component id for K3.

Mounting holes were adjusted to a standard 300x300 mil spacing from edges. (When "Velcro Slots" were added in REV-J3, the mounting holes were adjusted without concern for consistent edge clearance)

In REV-J3, Velcro slots were added to the board to allow securing amphenols. This involved moving the entire 1A2 signal bus around to accomodate. The board's width was decreased by about 1/4" by optimizing component positions and traces. (Smaller board widths simplify shipping) Electrically this version is no different from the previous REV-J2 or REV-J1.

In REV-J2, "EXTRA" (C9) had extra pads added to accomodate large components. For single board installations, this position is populated with a power resistor. Some small silk screen changes were made for part#s. Electrically this version is no different from the previous REV-J1.

In REV-J1, Northern Telecom LOGIC 10/20/30 support was made official; The buzzer circuit's polarity was swapped to support these phone sets. Small trace routing changes were made to "clean up", including improving clearance. Some small silk screen mods.

In REV-J:

Support for Northern Telecom phones was added. This involved reversing the polarity for the A leads for lines 1, 2 and 5. The changes for Line 1+2 were minimal, but the ICM modifications involved dropping the TIP32 (Q6) and its two resistors (R25 and R27), and replacing them with a single 1uF capacitor and some trace rewiring. In REV-J1 the buzzer leads were swapped on all the Amphenol connectors, to prevent an NT phone from blowing the board's fuse if directly connected without any rewriring on a connection block. NT LOGIC 10/20/30 phones are properly supported in REV-J1 (and up) if proper connection block rewiring is used; see the manual.

A missing ground in REV-H1 and REV-H2 was fixed; on EXT2, the lamp ground for Line 4 was omitted, causing its lamp to not light. This problem is only seen when 2 boards are interlinked. The fix for REV-H1 and REV-H2 is to add a jumper between pins 37 and 40 on EXT2.

The following are animated gifs showing the physical changes between older and current board revisions.

Documents

User Documents
User's Manual	User's Manual Covers Installation, Use, Troubleshooting, Equipment, etc.

Technical Documents
BOM / Parts List	Bill Of Materials: Digikey parts list with prices + total parts cost
PCB Layout File	The editable Sprint 6.0 PCB layout file (used to create Gerbers for printing PCB boards)
PIC .hex files [CPU1] and [CPU2]	Separate binaries for the two PIC chips, compiled from the V1.4 github source code
ExpressSCH Schematic (.sch)	The editable ExpressSCH schematic file
Circuit Description	Description of how the REV-J4 circuit works in the various 1A2 modes (Idle, Call, Hold, etc). There's an older circuit description for previous revision boards.
Gerber/Drill Files (1a2-REV-J4.zip)	Gerber + Drill files as a zip file
How to manage this project	How to load the PCB layout, save Gerbers, print the PCB boards, etc
PIC Programming Notes	My loose notes while learning to write the PIC firmware for this project

Firmware

As of board revision REV-J4, I switched to using V1.4 of the firmware which has the synchronized ringing feature, and redefined the data communication over the SYNC_ILINK pin of the interlink connector. So boards running V1.4 firmware can only be interlinked with other V1.4 firmware boards.

The older V1.3D firmware can still be used on the REV-J4 and older boards back to and including REV-G. The newer V1.4 firmware will ONLY run on REV-J4 boards. To use V1.4 on older boards, a trace has to be cut; see the "V1.4 Firmware Trace Cut" section in the circuit description for more info.

Up to and including REV-J3, I'd been using the same V1.3D firmware for all revision boards since and including REV-G. (Somewhere around serial #0055 I switched from V1.3C to V1.3D firmware to fix a subtle ring cadence problem.)

The firmware consists of two C programs, one for each PIC (cpu1 and cpu2) that are compiled using Microchip.com's free MPLAB X programming environment/IDE and their $50 "PICKit4" USB programmer:

1a2-pic-cpu1-firmware.c -- CPU#1 firmware that handles 1A2 logic functions on github
1a2-pic-cpu2-firmware.c -- CPU#2 firmware that handles Rotary Dialing on github
Debounce.h -- Function and struct to handle switch contact debouncing
README-debounce.txt -- Describes the debounce/hysteresis algorithm used to remove switching noise
Firmware Logic Diagram -- logic diagram of state machine for the 1A2 line management

To install firmware on PIC chips, you don't have to compile from source, you can use the .hex files above and use a variety of PIC programmers to put these hex files onto the PICs.

Or, you can compile the source yourself and program it onto the PICs using MPLAB X. In the MPLAB X IDE, you can create two projects for the PIC16F1709 chip; one for CPU1, and one for CPU2:

1. Drop these files into the CPU1 project:

2. Drop these files into the CPU2 project:

Build and "program" each project separately to burn it into the PIC chips.

The board logic functions are managed by CPU1 (IC8) include detecting incoming calls, flashing the lamps, handling ringing/buzzing for incoming calls, bidirectional data communication over the INTERLINK connector via the ISYNC_ILINK pin, and detecting line pickups and hold conditions.

The other PIC chip, CPU2 (IC10), handles the intercom features only (Line #5), including parsing rotary dialing, assists with DTMF decoding, and buzzer signal generation. The firmware for CPU2 is a relatively small program.

The software has no dependence on external libraries; it's entirely self-contained.

Board Layout And Schematics

Sprint Layout 6.0

You don't need this software to print these boards yourself; you can just use the Gerber + Drill files to have them printed.

You only need the software if you want to make changes to the board. I found Sprint6 very easy to work with, as you can "just draw it". Has a pleasant user interface and is very stable.. it never crashed once in the several months I've been working with it. And one can generate Gerber files from this software, allowing boards to be printed just about anywhere in the world.

I think the software cost me about $60 USD for a license, but I think you can use the eval to open and do everything but save. $60 is cheap, compared to what most PCB layout software costs. So with this program you can load up the board's PCB Layout File to edit it, generate Gerber/Drill files, and send for printing. I used these instructions for saving out the Gerbers and having the boards printed.

Sprint Layout: (Updated: 09-11-2021,02:15:05)

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