1A2 Multiline Phone Control Board - Rotary/Interlink/Pic, REV-F
[Update 05/14/19] Received the F1 boards from the printer today, and soldered one up to test the
Rotary vs. DTMF intercom dialing. Here's the result:
Here's a video where I give the new board a quick test of the new Rotary dialing feature.
This is a work in progress. There are several versions of this board now:
- REV-G1 is the newer revision, letting one combine two boards easily, and supports Rotary phones (as well as Touch-Tone) for intercom dialing
- REV-F (shown here, below) lets one combine two boards easily, and supports Rotary phones (as well as Touch-Tone) for intercom dialing
- REV-E (older) uses a PIC chip to simplify board construction where I can construct these in small quantities
- REV-B (oldest) uses all analog components. Nice design, but hard to build (many components)
History of Revisions
I started with an all-analog board, REV-B, and purposefully avoided using any CPUs in that design.
But when people started asking if I could sell them this board, I realized it was too hard to build
these in any quantity. So I went back to the designing board to simplify the circuit, and came up with
REV-E, which replaces about 50 discrete components with a single chip CPU ("PIC" chip). Later, someone
requested I resurrect a bridging feature that lets one easily combine two boards to double the line and extension count,
and also asked if rotary phones could be supported for intercom dialing. So I came up with REV-F which adds those
features. That revision necessarily needed TWO cpus to properly handle all the features.
Revision F and F1
The benefits of using a CPU in THIS version of the board allows for fewer parts,
smaller board, and less labor during assembly, all of which keeps the final board cost
and build complexity down.
New in Revision F (REV-F), there are now 2 PIC cpus onboard; one to handle 1a2 logic,
the other to handle the new support for Rotary Dialing (and replaces a TTL chip in the older designs).
Also, an "Interlink Connector" allows two boards to be easily combined with a simple ribbon cable
to provide a total of 4 CO lines, and 8 extensions that can all be uniquely dialed on the intercom line
via Touch-Tone or Rotary. This page shows how to interlink two boards.
"PIC PROGRAM" sockets allow re-programming the PIC chip's firmware on-board,
without having to remove it, using Microchip's own PICKit 4 USB chip programmer, a $50 device.
This is the software that manages the logic functions and rotary dialing functions of the board.
This is a C program compliled and uploaded to the PIC chip using Microchip.com's XC8 C Compiler,
part of their free MPLAB X programming environment/IDE:
The board functions managed by the 1A2 logic PIC chip (CPU#1) include detecting incoming calls, flashing the lamps,
powering up and directing the external ring generator, detecting line pickups and hold conditions,
and handles some intercom dialing detection and supplies the 60Hz extension buzzing signal.
This software uses only 5% of the 1K of data memory, and 17% of the 8K program memory.
The Rotary Dialing PIC chip (CPU#2) handles rotary dialing, and emulates a 7445 for 1-of-8 decoding.
The firmware for CPU2 is a relatively small and simple program.
Board And Schematic
I used Sprint Layout 6.0 to create
and edit this board. I found it very easy to work with, as you can
"just draw it". Has a pleasent 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 gerbers + drill files, and send for printing. I used these instructions
for saving out the gerbers and having the boards printed.
Sprint Layout: (Updated: 05-21-2019,04:35:49)
These are the schematics for the circuit board in its current state:
Schematic Sheet Guide
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