The Mitel SX-20 PABX

The Mitel SX-20 is a 72 line / 8 trunk Private Automatic Branch eXchange (PABX) from the early 1980s, designed and built in Canada. It features:

• 6809 processor running two banks of 48kB EPROM each (read about bank switching)
• analog crossbar (Mitel's own MT8804 chips, also designed and manufactured in Canada)
• dual DTMF decoders (Mitel's own hybrid chip design)
• POTS or SuperSet phone support
• SMDR output
• RMATS port

I have one running the phones in my house, and a lot of spare cards.

Click on the images below for more information, or the XL for the full size image.

Architecture

This diagram is a schematic representation of the overall layout of an SX-20 system.

The SX-20 System XL

The Hardware

The SX-20 consists of a CPU card, a Miscellaneous card, and a number of line cards and trunk modules.

The CPU II

The SX-20 CPU card hosts a PROM card and an optional Remote Maintenance, Administration, and Test System (RMATS) card.

The CPU II Card XL

The EPROM / RAM Card XL

RMATS Card XL

The Misc Card

The miscellaneous card hosts up to four trunk cards (serving two trunks each), and one or two DTMF receiver hybrids.

The Misc Card XL

Mitel DTMF Hybrid XL

Trunk Card XL

New Display XL

Line Cards

There are two types of line cards; ones that interface to Plain Ordinary Telephone Sets (POTS) and ones that work with Mitel's SuperSet phones.

The Line Card XL

The SuperSet Line Card XL

The Backplane

The Backplane XL

The Remote Display

Remote Display XL

Insides XL

Test and Development

During test and development, I built a three-layer "cathouse" that selects one of the 24 lines:

SX-20 Cathouse XL

In order to simplify new software development, I created a 6809 emulator using a Raspberry Pi PICO that replaces the 6809 on the CPU card:

PICO-Based 6809 Emulator XL

E and Q Signals XL

This is the bench setup during development. Note the power supplies! The SX-20 takes -24V, -8V, +5V, and +7V (as well as 90V ringing, but I didn't supply that).

SX-20 Development XL

Documentation

The SX-20 Generic 503 manual

PN9102-050-006-NA Issue 3 Mitel Standard Practice Generic 503

Source code

********************************************************************
*
*          PP POWER UP
*
********************************************************************
*
*  SET UP EXORCISER INTERRUPT VECTORS
*  IF MBUG EXISTS
*    THEN INITIALIZE IT
*  ENDIF
*  WRITE ALL EIGHTS TO LOCAL & REMOTE DISPLAY
*  CALCULATE PP PROM CHECKSUM (0)
*  CALCULATE CP PROM CHECKSUM (1)
*  TEST VOLATILE RAM (EXCEPT MBUG RAM) (2)
*  ZERO VOLATILE RAM (EXCEPT MBUG RAM)
*  INITIALIZE WORK AREAS
*  INITIALIZE COMMAND BUFFER FREE LIST
*  INITIALIZE HARDWARE
*  BLANK ALL DISPLAY
*  DETERMINE CONSOLE TYPE
*  RUN CP (CALL PROCESSING)
*
PPPSTR  EQU     *               START OF PP PROM
CHKSUM  FCB     0               STORE MINUS CHECKSUM HERE
        FCB     PPREV           PP REV #
        FCB     PPBLD           PP BUILD #
        FCB     PPPTC           PP PATCH #
*
PWRUP   EQU     *
        STA     NVREN2          SELECT PP RAM PAGE
        LDS     #PPSPTP         INITIALIZE PP STACK POINTER
        SEI
        LDA     MBPI.
        CMPA    #$BB            MBUG EXIST?
        BNE     PWR00           NO
        JSR     NITMB.          YES INIT MBUG
        SEI
*       JMP     PWR30           SKIP ALL THE TESTS
*
*  OUTPUT EIGHTS TO LOCAL & REMOTE DISPLAY
*
PWR00   LDA     #$01
        LDB     #255
PWR01   EQU     *
        STB     DISLOC
        STB     DISREM
        STA     DISREG
        STA     DISREG
        STA     WDOG
        DECB
        BNE     PWR01