GPT marketed this system as the DXE whilst Mercury Communications called it the Trinity.


GPT MASTERLINE DXE is a new concept. It is more than just a keysystem, it is a totally DIGITAL communications package.

It gives the user combined voice and data traffic all in one system. Telephones, word processors and personal computers can be connected by the same system wiring, thereby offering a high level of integration of services.

The DXE builds upon the pedigree of its successful predecessors. It not only offers existing facilities but brings new and more advanced features to the user.

The DXE does not stop at just providing new features, it radically changes the way systems connect.

Historically, telephone systems have used 'star' wiring for extension connections. Since the DXE is a completely digital system from end-to-end, connections can now be made using both 'star' and 'bus' topologies. The use of bus wiring means neater, faster and easier installations benefiting both the user and installer.

No longer are terminals on the system restricted to just keystations. Any type of data terminal which incorporates a standard RS232 C1V24 interface can be connected via a PC data adaptor. Since the DXE is totally digital and does not differentiate between speech and data, calls can easily be set up between PC's for transferral of data.  

The DXE provides total portability of terminals. Terminals can be moved from socket to socket and still retain their attributes, enabling great flexibility with minimum disruption.

Flexibility exists not only in the features and facilities that the DXE provides, but also in its hardware design. The system is available as a 4+20 and a 24+70 but can be configured as a 4+10 and 12+40 respectively due to its modular design.

Since some facilities are not required with small configurations, for example operator working, differences exist between the 4+20 and 24+70 and these will be indicated by (#) throughout the document.

Technical Description

The MASTERLINE DXE is a highly sophisticated keysystem. As well as having digital switching control, all transmission around the system is also digital.

DXE offers more features and services, including the following:

  • Voice and data integration

  • Bus and Star wiring

  • Voice messaging (#)

  • Text messaging

As well as extensive voice features the system has the capacity to provide a Local Area Network (LAN) by connecting any RS232C/V24 devices (personal computers, printers) via a data interface. A built-in modem (#) is available for external communication which is compatible with CCITT V25bis standards.  

DXE can be wired as either a bus or star system or by a combination of both.
Star wiring is the traditional way of connecting terminals. Each telephone needs its own separate connection to the central unit.

Now bus wiring is possible since the system is totally digital. It allows a single two-pair connection to be utilised by multiple keystations. With bus wiring an unlimited number of sockets can be provided, with the proviso that only a maximum of ten keystations can be connected to a single bus at any one time 

Bus wiring offers the following advantages over that of star:

  • simplified and quicker installations

  • fewer cable runs

  • easier portability of terminals around the system

  • easier fault tracing

Star wiring may be used where it already exists for previous systems or where it may be more suitable in certain office environments. DXE allows a combination of both wiring types so that the advantages of each may be combined when appropriate.         

The table below show the capacity in the 4+20 and 24+70 and sub-equipped 12+40 systems for the various facilities available.      


4 + 20

12 + 40

24 + 70

External Line








Handsfree Circuit




Extension Conference Circuit




External Paging




External Music on Hold




Background Music Terminal





Size and Weight                                                                                                               

W (mm) L (mm)  H (mm)   Weight (Kg)

4 + 20


Main Equipment

588 270  81 5.2

Expansion PSU

175 270 78 1.8

Telephone Set      

195 223 76 0.75



12 + 40, 24 +70


Main Equipment 

344 325 410 8.2

Power supply 

344 325 193 11.7

Expansion Cabinet   

344 325 193 11.7

Telephone Set            

195 223 76 0.8

DSS Console

195 223 60 0.6



All systems


Analogue Adaptor

96 164 49 0.4

PC Adaptor

192 210 49 1.2

DSAP Console

195 223 60 0.6


99 193 35 0.2

Systems Cabinet
makes full use of ASIC technology in order to provide a compact package. It has been designed around a bus structure with universal card slots, this enables maximum utilisation of space.

Keystation Terminals
There are two types of keystation available. A display station, which displays time and date, number being called, internal number calling in and text messages. It also has a timer to show duration of calls and a clock showing two alarm settings.
Absent text messaging is available (selectable from a menu of nine options) on display keystations without any extra hardware. The text is left at a station and is sent to the internal calling party.
A standard station which has access to all the facilities of the system apart from those requiring a display Although standard terminals cannot receive text messages, they can set them to be sent to calling display terminals.

DSS Console
Direct Station Select (DSS) consoles can be fitted to the system. These enable the user single button access to any station. It also provides a busy status lamp field for all the stations.

Automatic Operator Console (DSAP)
The DSAP plugs into system wiring and enables direct inward dialling (DID) on trunks. An incoming call is answered by the system, a prompt is then returned to the calling party. Digits dialled in MF will then route the incoming call to the specific station. If the internal station is busy or not answered in a specified time, or a wrong number is selected, then the call is re-routed to a pre-programmed station. Up to a maximum of 8 stations can be pre-programmed to receive failed calls.
Calls can be transferred on ringing so the DSAP will be available to answer other external calls.

Personal Computer Data Adaptor
PC data adaptors provide an interface to the system for data terminals. An RS2321V24 port connects the data terminal to the PC adaptor, which in turn connects via a standard telephone plug on to the system wiring.
Data terminals can set up calls between each other and transfer information at up to 9.6 kbits/s. One data terminal can also broadcast the same information to a group of other terminals simultaneously. A call can also be set up between a display terminal and a data terminal. The data terminal sends text messages which are then displayed at the keystation.
A pooled modem card (#) installed in the system will extend this Local Area Network facility via the public switched telephone network using CCITT V25 bis standards.

Simple Telephones
An analogue port adaptor will allow two-wire LD/MF telephones to be connected to the system. This will permit many facilities of the system to be accessed via a simple telephone.

Door Unit
One or two door units can be associated with the keysystem. When the door phone button is pressed, a chime tone is heard at a preset station enabling a conversation with the visitor. Control of an electric door lock is also available which can be used in conjunction with the doorphone.

Equipment 12 +40, 24+ 70
Dependent on the size and configuration of the system, various items of equipment are required. There are however mandatory items which must be provided. These are a power supply main equipment cabinet, CPU card and a number of station and trunk interfaces. The rest of the system cards will be determined by the particular application, for example if external data communication is required a pooled modem card is then needed.

Certain cards take up universal slot positions. The main equipment provides eight universal slots, if however additional slots are required an expansion cabinet facilitates a further eight. Two such cabinets can be provided, enabling a maximum of twenty four universal slots to be used.

Following is a list of equipment with the item name, a brief description and the maximum number of the item that can be fitted to each system. For equipment codes refer to the relevant proforma.  

When a trunk is connected directly to the PSTN, the line loss should not exceed 10db.
When trunks are connected to a PABX and PSTN calls are made via the PABX, the line loss between the PABX and PSTN should not exceed 8db.
DTMF or LD dialling is assigned to each trunk by system programming.
Loop-disconnect pulse speed and ratio is 10pps/33% make.
Under normal operation power for the trunk call speech path is derived from the system power supply.
Under power-fail conditions, power for the speech path of fallback stations is derived from the PSTN or PBX.
The 'ringer equivalence number' for each trunk is 3.

Operating environment

  • Operating ambient temperature range, control unit and keystations: 0'C to 45'C.
  • Storage ambient temperature range: -20'C to 70'C.
  • Humidity range: 10% to 95% non-condensing.

Approval no NS/3343/23/L/602027

The Mercury Trinity DXM 24/70 Digital Keysystem comprises two parts:

(i) Apparatus which is type approved and is specified in the PXML.

(ii) Apparatus consisting of an individual personal computer under the terms and conditions of OFTEL's General Approval, Number NS/G/1234/J/100003, providing screen-based console and Message Master features.

The Mercury Trinity DXM Digital Keysystem uses 4 Wire Digital Communication between the main equipment and all system terminals. The system uses Mu-law analogue to digital encoding in a 32 channel Pcm architecture.

All terminals can be interchanged between extension line sockets allowing terminal facilities/class of service and extension numbers to remain with the terminal rather than with the extension interfaces.

The system also has the facility to connect extension wiring as a local area network where one 4 wire cable can feed up to 10 System Terminals as an alternative to the conventional 4 wire STAR type wiring.

Connection to the NTP is made via the TJF. The TJF consists of labelled Krone connectors contained within a 300A type MDF box. Krone connectors are provided for extension wiring, NTP termination and power fail telephone connection.

Under "power fail" conditions the system will cease to function.  Power fail telephones provide the only means of originating or
receiving calls during "power fail" conditions.

In the event of a major equipment failure the main equipment must be switched off. The power fail telephones will then provide the only means of originating or receiving calls.

The Mercury Trinity DXM 24/70 Digital Keysystem consists of a power supply unit and a main cabinet.

Expansion cabinets are available to allow the installation of additional cards above those in the basic main equipment. A maximum of 2 expansion cabinets can be installed onto the 24170 equipment.

The power is provided to the Mercury Trinity DXM 24/70 Digital Keysystem by a flexible mains supply cord via a standard 13 Amp mains plug.

The system supports a number of different types of key telephones (Integrated Terminating Stations). A 2 Wire analogue telephone port is provided via an analogue adaptor. The analogue adaptor takes the position of a system terminal.



Trunk/Extension Min Max
PSTN Loop Calling424
Private circuits AC15A/DC5 Tie Lines012
BS 6317 Analogue Telephone Extensions069/70
Integrated Terminating Stations170

Circuits Per Interface Card

Card Name Type Card Description No. of circuits
DXP12 COU-ATrunk Card with Meter Pulse Detection and Grounding4
DXPI2 COU-BTrunk Card without Meter Pulse Detection and Grounding4
AC15AC15 Daughter Card1
DC5DC5 Daughter Card1
DXP STU-D10 Station Bus I/F Cards10 Terminating Stations
DXP STU-A10 Station Star I/F Cards 10 Terminating Stations
DX.M AEAPAnalogue Adaptor1

Interactive Quantities
Essential Hardware

Description Quantity
DX.M 24M-Vl1
DX.M P24P-Vl1
DX.M 24 MCU-C1
DXP STU-DTotal of 1 -7
DXP STU-ATotal of 1 -7

Optional Hardware (Main Equipment)

Description Quantity
DXPI2 REJU-A (Per key telephone)1
AC15 Daughter Card12
DC5 Daughter Card12

option Cards

Description Quantity

NB: Any combination of option cards may be installed. The maximum quantity stated must not be exceeded. There are eight slots available for option cards. An additional eight slots are available with the addition of every expansion cabinet (up to a maximum of 2 cabinets) .  Examples of interactive quantities are given below.

DXP12 COU-A DXP12 COU-B DXP12 TLU-A Additional option Cards
Additional  option Cards
cab.1 Exp.
Additional option Cards
2 Exp. Cab.
60 0210 15
06 0210 15
10 619 17
01 619 17
00 6210 18
00 0816 24
11 0614 22

With a DXPI2 TLU-A Card fitted, the following tie line card quantities may be fitted:

AC15 Daughter Card DC5 Daughter Card

System Terminals
The following hardware takes the position of a system terminal!

Description Max Quantity
DX.M 4 OW DSS 12
DX.H 4S 70
DX.M 24E 70
DX.M 24ED 70

Supplementary Information
For each PSTN line there shall be at least one key telephone station.
A maximum of two extensions can be in conference with a single PSTN line.


Facilities available to operator and integrated terminating facilities:

Answer an Incoming PSTN Trunk
Call Diversion
Call Waiting Indication
Camp-on Exchange Line to Busy Extension
Do Not Disturb (DND)
Dual Tone Multi Frequency (DTMF) Signalling to the PSTN
Extension to Outgoing PSTN Trunk
Group Pick Up
Handsfree (Loudspeech)
Headset Operation
Hold (Internal and External Source)
Last Number Redial
Loop Disconnect Signalling to PSTN
Loudspeaker Tone output
Memo Dialling
Mercury Least Cost Call Routing
Outgoing Routing Restriction
Personal Abbreviated Dialling (10 Personal Number@l
Pick Up
Recall (Timed Break and Earth Recall)
Subsidiary Operation (Behind PABX)
Voice Message Answering an Incoming Call

Features available at DSAP

Direct Inward Dialling (DID)
Ring Inward service at DID

NB: For details of operation with respect to the facilities listed above refer to system documents.

a) Mercury Trinity DXM Keysystem User Guide Executive Display, Executive and Standard Instruments
b) Mercury Trinity DXM Simple Telephone User Guide
c) Mercury Trinity DXM System Manual

Unapproved Facilities

Class of Service ions
Each terminal is individually identified and portable and carries its class of service with it. The class of service route restriction for each terminal can be programmed in system programming. (See Section 2.29.4of Mercury Trinity DXM system manual.)

Standard TerminalExecutive Terminal (24 button)
Std terminal (4 button)Executive terminal (24 button)
Made for Mercury Communications
Automatic Operator ConsoleDirect Station Display Console
Analogue AdaptorDoorbox

DXE Socket Wiring
Star wired

LJU 2/3C Wire colour MDF termination (left to right)
3 Blue/White 2
2 Orange/White 4
5 White/Orange 3
4 White/Blue 1

Bus wired

LJU 2/3C Wire colour DDK terminations
3 Blue/White 4
2 Orange/White 2
5 White/Orange 1
4 White/Blue 3


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Last revised: November 05, 2022