PAGE No. 55

The Minirax Mk II
July 1964

A new miniature 24-volt rural automatic exchange is described. It has capacity for 20 subscriber lines and presents the attractive features of small physical size, ready mobility and dual purpose. It can function either as a dependent exchange with up to three junctions, physical or carrier, or as a discriminating satellite to allow inter-calling with other automatic exchanges and access to the nationwide network if required. Its in-built battery can be charged over a physical junction from a remote location thus making the Minirax completely independent of a mains supply.

SOME years ago the Company introduced the Minirax, a miniature automatic exchange arranged to meet the specialized needs of small rural communities. This 20-line exchange, designed for unattended operation as a dependent exchange, provided fully automatic local service and direct bothway working with a parent exchange, manual, auto-manual or automatic. Its power was derived from a small-capacity 24-volt battery trickle charged over one or more physical junctions from the parent exchange.

Recent years however have seen a phenomenal increase in the demand for direct subscriber dialling of calls outside the home exchange. In some areas the requirement is merely for the inter-dialling of calls between a group-centre exchange and its dependents; in others, a nationwide plan. In the next few years it is almost certain that facilities allowing subscriber dialled international calls will be demanded.

In order to permit the telephone subscriber in a remote area to enjoy similar facilities to those offered by large public exchanges, it was decided to introduce the Minirax Mk II which, whilst maintaining all the facilities of the original design, would allow its integration with a nationwide subscriber dialling scheme.

A fundamental requirement for such a scheme is a nationwide numbering plan in which all the subscribers have a unique designation which nevertheless is similar in form to that of all other subscribers connected to the network. With this arrangement, all operators and subscribers wherever connected must use this designation as a destination code to reach the required subscriber through the dial-switching network. All exchanges therefore become part of a large multi-exchange area with each exchange having its own distinctive identity for routing purposes. Complementary to this numbering plan is the need for selection by the equipment of the appropriate charging rate on calls to various destinations.

With these considerations in mind the Minirax Mk II has been designed to operate either as a dependent exchange in a similar manner to the original design or as a discriminating satellite, thus obviating the need for special routing codes. Its ability to assume either of these roles makes the new exchange suitable for an Administration not requiring subscriber dialling initially but contemplating its inclusion at a later date.

Conversion to discriminating satellite working merely entails the adjustment of straps in the equipment and the substitution of appropriate jack-in junction relay sets where necessary, dependent upon the type of junctions and metering arrangements existing at the particular exchange.

The features listed below can all be catered for by the exchange whose design flexibility allows equipment for some of the facilities to be fitted or omitted to suit particular requirements:-

Automatic connection between subscribers on the Minirax.

Automatic connection from the Minirax to a parent exchange and to a neighbouring Minirax if required.

Physical or carrier junctions to parent exchange.

Alternate testing of 1st and 2nd choice junctions.

Coin-box (pay station) discrimination with visual or audible signal to parent exchange operator.

Metering equipment (single fee, multi-fee and periodic).

Manual hold.

Visual and audible fault alarms.

Visual indication of a fault condition extended to a panel in the Minirax building or in an adjacent building.

Alarm test circuit.

Ringing and tones with periodicities to Administrationsí requirements.

Battery charging over junctions.

The equipment functions satisfactorily on subscriber lines and junctions having a maximum loop resistance of 500 ohms including the telephone instrument, and an insulation resistance of not less than 50,000 ohms. The exchange is designed to consume the minimum of current and to operate with a variation of battery voltage between 22 and 28 volts.

Figure 1 - Simplified diagram of connecting circuit

A 2-digit numbering scheme 20 - 39 is used for all subscriber lines when the Minirax functions as a dependent exchange. Access to an outgoing junction is by dialling a single digit which may be 8, 9 or 0.

For discriminating-satellite working, these local numbers are prefixed by two or three digits to conform to the numbering scheme of the area. Seizure of a junction and the switching equipment at the parent exchange is immediate on pick-up.

Four connecting links are provided each with strapping arrangements for adaptation to either method of working. Each link (see Figure 1) comprises a linefinder and a connector switch both of the single motion 25-point type. Subscriber line circuits are connected direct to the linefinder bank contacts and multipled in corresponding order to the bank contacts of the connector switch which functions as a combined group and final selector. Incoming and outgoing sections of the bothway junctions are connected to the early outlets of the linefinder and connector switches respectively and trunked to line via relay sets.

For all calls, local and junction, a linefinder (LF) in the link circuit is allotted to search for and find the calling condition.

Incoming Junction Call
When an incoming call is received over one of the three junctions the wipers of LF are arrested at the appropriate outlet (1, 2 or 3) and an operate circuit is completed for relay LC. As a result, the connector switch CS self-drives to outlet 12 and relay CD operates to prepare the dialling circuit for the acceptance of the two local digits from the incoming-junction caller.

If the first of the two digits of the wanted number commences with 2, CS steps to the appropriate outlet under control of the dial. As it steps on to outlet 14, relay TH operates to release LC and re-operate CD, thus allowing the switch to respond to the final digit and step to the called-subscriber outlet (i.e. 15 to 24).

Similarly, when digit 3 is dialled as the penultimate digit, CS steps under control of the dial to outlet 15. On reaching this position, relay SH operates and the switch then self-drives to outlet 4. Relays TH, LC and CD function again, as described above, to position CS under control of the final digit on the appropriate called-subscriber outlet (i.e. 5 to 14).

At this stage the called line is tested for three conditions; (a) free, (b) busy and (c) spare connection. Busy and NU tones are transmitted for (b) and (c) respectively. In addition, NU tone is applied to the calling line when a penultimate local digit other than 2 or 3 is dialled.

Because of the need to maintain current drain at the minimum for this type of exchange, only four relays are held operated in the connecting link during the conversational period.

Local and Outgoing Junction Calls
When a call originates from a local subscriber the linefinder drives over outlets 1ó3 and consequently relay LC does not operate. When the linefinder cuts-drive on the appropriate outlet, CS searches over the junction and overflow outlets. On seizure of the junction, dial tone is transmitted to the caller and all relays release with the exception of HA.

As the caller dials the wanted subscriberís number following the receipt of dial tone, the digits are repeated into the parent exchange and also into the discriminator associated with the junction circuit. If the digits dialled correspond with the exchange code of the Minirax, an earth is returned over the K lead; otherwise the call remains routed through the parent exchange.

When earth is returned, relay LC operates, CS drives to outlet 12 and the junction circuit is released. Thus conditions are established for completing a local call as described under íIncoming Callsí.

Should all junctions be busy, an overflow circuit connected to outlet 4 is seized, enabling the local call to be completed.

For dependent working, no differentiation is made between calls originated locally or incoming via the junctions; relay LC operates on both types of call after the linefinder has found the calling line. In both instances therefore, CS drives to outlet 12, CD operates and the function of the circuit for the acceptance of the two local digits as well as the testing of the called line is identical to that described for discriminating satellite working.

When the single digit (8, 9 or 0) is dialled for an outgoing junction call, relay DX operates and CS is driven to the home position. This causes the release of the LC relay and the subsequent operation of CA to drive CS in search of a free junction. On seizure, relay HA is maintained held and all other relays in the link circuit release, further digits being repeated over the junction to the parent exchange if necessary.

All junction circuits irrespective of classification, i.e. manual, auto-manual or automatic, can be supplied to function over physical lines or over line or radio carrier. Line-carrier equipment (RC101 Rural Carrier) can be incorporated in the Minirax.

For complete reliability of operation, the circuits are so designed as to eliminate all possibility of false metering, especially in the event of simultaneous junction seizure at both exchange terminals. In addition, the first and second-choice junctions are arranged for alternate seizure by the calling subscriber during periods of light traffic. This arrangement not only permits uniform distribution of traffic over the two junctions but, more important, it ensures that if a fault occurs on a junction, a calling subscriber is not isolated from the junctions by seizure of the same faulty junction on successive attempts at originating a call. A further design facet of interest is that an incoming junction call can still be completed even though the outgoing side of the circuit is held by a subscriber who has dialled a barred service.

Equipment can be associated with auto-auto junctions to determine the treatment (i.e. barring and metering) to be applied to various types of call. Three classes of subscribers are catered for, ordinary, restricted, and coin-box. Coin-box subscribers are barred from chargeable services and calls to the nationwide network. Restricted subscribers can be denied access to STD on a temporary basis under control of a lock and key in the telephone.

Table 1 shows a typical range of metering and barring facilities applicable to the discriminating satellite and dependent systems.

The additional coin-facility referred to in the table is one which enables a line unbalance to be applied from a coin-box when a further coin is inserted. This allows the metering equipment to recognize that further units of time have been purchased.

Any subscriberís line circuit can be arranged for coin-box working by appropriate terminal strapping. Calls to the operator from these lines can be recognized either by a different calling lamp or by the application of a burst of distinctive tone when the operator answers. This tone can be checked by removal and re-insertion of the operatorís plug.

This feature is provided to ensure that common switching equipment is not held up unnecessarily owing to a 1oop or earth fault condition. If a subscriber fails to dial after lifting the handset or omits to clear down the connection at the termination of a call, the switching equipment is forced-released after 30 seconds, and the calling line locked out as long as the condition persists.

Types of Call
Types of Metering Method of
Ordinary Subscribers
Coin-box Subscribers
without additional
coin facility
Coin-box Subscribers
with additional
coin facility
(D.S.R. system only)
Local Calls, i.e.
within the Minirax
Unit fee untimed Unit fee untimed Unit fee untimed D.S.R. or Dependent
Interurban Calls,
i.e. junction calls where meter rate is same for all routes.
Unit fee untimed Unit fee untimed Unit fee untimed D.S.R. or Dependent
Barred Barred
Unit fee on answer and thereafter repeated at intervals of 1, 2, 3, 4 or 5 minutes.
Unit fee timed out after interval determined for calls from ordinary subscribers,
Unit fee repeated after interval determined for calls from ordinary subscribers if further coin is inserted, otherwise call is released. D.S.R.
D.S.R. or
Multi fee untimed, 1-6 pulses.
Unit fee timed out after arbitrary interval of 1, 2, 3,4 or 5 minutes.
Unit fee repeated after arbitrary interval of 1, 2, 3, 4 or 5 minutes if further coin inserted, otherwise call is released. D.S.R.
Barred Barred D.S.R. or
Multi fee, l-6 pulses, on answer and thereafter repeated at intervals of 1, 2, 3, 4 or 5 minutes. Barred Barred D.S.R. or

 Types of Call

Types of Metering

Method of
Ordinary Subscribers Coin-box Subscribers
without additional
coin facility
Coin-box Subscribers
with additional
coin facility
(D.S.R. system only)
Interurban calls
where meter rates
depend on route.
Unit fee on answer and thereafter at intervals of 1, 2, 3, 4 or 5 minutes appropriate to route (determined at parent exchange and repeated over the junction to the Minirax). Unit fee timed out after compromise interval of 1, 2, 3, 4 or 5 minutes on all routes. Unit fee repeated at compromised interval of 1, 2, 3, 4 or 5 minutes on all routes if further coin is inserted, otherwise call is released. D.S.R.
Barred Barred D.S.R. or
Calls to nationwide network. Time and Zone metering, i.e. single pulses repeated at intervals determined in the register equipment at the trunk or zone centre, and returned over the junction to the Minirax. Barred Barred D.S.R.
Calls to chargeable special services,
Unit fee repeated at intervals determined in the register or other equipment at the trunk or zone centre and returned over the junction to the Minirax. Unit fee timed-out
after arbitrary interval or that interval used for interurban calls
from ordinary sub-
scribers determined at Minirax.
Unit fee repeated after arbitrary interval or that interval used for interurban calls from ordinary subscribers if further coin is inserted, otherwise call is released. D.S.R.
Barred Barred

Fault conditions are indicated by a lamp display and also by tones which are obtained by dialling an alarm-test number either locally or from a distant exchange.

Alarms may be extended locally, if required, to operate a buzzer in a nearby room. Operation of the buzzer cut-off key illuminates a lamp display at the remote point to indicate the nature of the fault.

The l2Ah capacity of the 24 volt battery is larger than that of the original design, but similar arrangements have been made for charging the battery over physical junctions during the non-conversational time. Where a combination of carrier and physical circuits is provided, the physical circuit is arranged to be taken into use last.

Three junctions will carry approximately 0.6 erlangs at a grade of service of 1 in 50 and the traffic carried by the third trunk under these circumstances is 0.048 erlangs. With a day-to-busy hour ratio of 8:1 the physical junction is occupied for a total of 0.384 hours per day and is therefore available for charging purposes for 23.6 hours out of every 24.

Should a local power source be available, a conventional charging scheme may be employed if desired, thus enabling the exchange to function entirely with carrier junctions in areas where land lines do not exist and all communication is provided by v.h.f. radio.

The various ringing and tone interruption and meter pulse are derived from transistor generators and pulse circuits actuated by a uniselector and self-interrupting relay chains.

Figure 2 - General view of Minirax
Mk II Unit
Figure 3 - Minirax Mk II Unit with dustproof doors removed

The cabinet (Figure 2), which houses the telephone switching and Rural Carrier equipment, is identical to that used in the original design but the mounting arrangements have been altered slightly to provide space for the additional equipment. Figure 3 shows the layout of the equipment in which the jack-in relay sets occupy two shelves, each with capacity for 6 units. The upper shelf accommodates the four connecting links, the ring, tone and meter pulse generator, and the alarm and miscellaneous equipment, including the overflow circuit if required. The lower shelf mounts three junction relay sets and three discriminators or route restriction and metering sets. Common shelf jack wiring enables the various types of junction relay set to be jacked-in in any of the positions.

The connector, linefinder and discriminator switches are of heavy-duty pattern and the relays are of minor, major and high-speed type. The circuitry is conventional and the components orthodox to ensure complete reliability.

The Minirax Mk II offers wide scope of application and is suited to meet the present-day telephone requirements of progressive rural communities. By its use, the desired parity with larger areas served by STD facilities can be fulfilled, either immediately or at an Administrationís convenience. Its small size and independence of a mains supply avoids the need of special site accommodation, and the use of dependable components and established switching principles ensures minimum fault liability.

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Last revised: October 15, 2019