Introduced around 1970 the Thorn Ericsson ARD561 had an extension capacity of 270, with a three digit numbering scheme.  The system was crossbar.

Capacity - Fully equipped
Extensions - 270
Exchange Lines - 40
Connecting Circuits - 20
Operator Positions - 3
Registers - 6
Enquiry Circuits - 6


L.M. Ericsson's private automatic branch exchange ARD 562 is designed on the register controlled bypath principle.
The only switching elements employed are relays and crossbar switches.

The telephone instruments used are ordinary sets with a dial, alternatively pushbutton instruments may be used if the appropriate exchange line relay sets and add-on equipment is provided.  The extension number can be three or four digit in length.

The exchange can be adapted for any public system of manual or automatic type.

The minimum economical capacity of the exchange is as low as 180 extensions with up to 10 exchange lines. By the addition of relay sets and racks the exchange can be extended to the full capacity of 540 extensions and up to 100 exchange lines.

Capacity - Fully equipped
Extensions - 540
Exchange Lines - 100
Connecting Circuits - 40
Operator Positions - 8
Registers - 14
Enquiry Circuits - 20

Internal calls are made in the same manner as in public systems, by dialling the required number. For outgoing calls a prefix digit is dialled, followed by the subscriber's number.

Extensions may be barred for outgoing calls to the public network.

Extensions which have access to the public network may be barred for automatic trunk calls. Alternatively with the addition of trunk discrimination equipment, extensions may have access to certain trunk routes only.

Incoming calls from the public network are received by the operator and transferred to the required extension.

All conversations are secret.

A number of optional facilities are available and are described in add-on facilities.

A maximum of 8 operator positions may be provided with a fully equipped exchange, each position can accommodate a maximum of fifty exchange lines.
Each operator position consists of an operators console and the operators relay equipment. The connections are established by means of pushbuttons, which incorporate lamp signals to indicate to the operator whether an extension is free or engaged, barred or open etc.

When the operators are off duty their consoles may be night switched. Incoming calls may be directed to one extension (common night switching) or to several predetermined extensions (individual night switching). Alternatively, incoming calls may be signalled by a bell system. A grading arrangement is provided so that the traffic may be spread evenly among the operators.

During an external call an extension may make an inquiry call to another extension or to the operator. In the inquiry condition the call may be transferred before or after answer.

A bothway operator's line is provided for the extensions to call the operator and for the operator to call the extensions. To call an extension the operator presses a key and keys the required number.

The relay sets and switching units are accommodated in racks which are interconnected by plug ended cables. The relay sets are connected to the rack wiring by plug and jack. This arrangement offers great advantages as installation, maintenance and extension of the PABX is simplified.

The minimum number of racks is three double and two single sided racks and the number of racks is increased until the ultimate capacity is reached.

The ARD 562 PABX operates on 48v d.c., but the voltage may vary between 44v and 54v.

The exchange can carry a traffic of 0.17 erlangs per extension with 50% internal traffic and 50% external with a congestion of 2 per min.

The following description is of a typical ARD 562 system and some details may vary from market to market. It should also be noted that some facilities may not be available in certain countries.

Standard switching units
BLR Attendant's service line.
FDK Relay set for connection of central office trunks to the attendant's position equipment. One or two FDK's arerequired per group of 10 trunk lines.
FDR-C Bothway central office trunk circuit. There is full availability to stations in all groups.
FDR-X Bothway tie trunk relay set.
FFR Inquiry circuit. The relay set is also used to set up an incoming call to stations during common and individual night switching and incoming calls via tie trunks (FDR-X). Two FFR's per 270 group are used as overflow SNR's.
FMT Attendant's position equipment. Each attendant's console (FMA) requires one FMT. The FMT contains a register for use with the attendant's keyset and control relays for lamps and keys on the FMA.
FMTI 1 Calls the correct 270 group marker, depending on the chosen hundreds digit.

MC Relay set for connecting switching units e.g. RR, REG-D, FMTII etc. to the B stage marker.
MP Alarm relay set which replaces SG R if external tone supply is provided.
MFA A-call marker which identifies the calling 270 group and switches the calling station to a register via SNR or and FDR-C to a register via F FR.
MFE A-call marker which selects free SLA, SLB links, free SNR's and FFR's and effects break-out within a group of SNR's etc.
MT Marker for test functions and for control of connections via other markers.
MTAI B-call marker which selects and controls break-out of free SLA, SLB links, transfers digit information from REG, REG-D, FMT and RR to SLM in the required 270 group. Also control relays for switching of selectors belonging to 270 groups 1 and 2.
MTAII B-call marker which indicates in which 90 group switching is to take place. Also free marks verticals available to SLC links tested in MTB.
MTB B-call marker which selects and controls break out of SLD, SLC-SLB link.
MTC Marker which connects free SLC verticals to MTB via MTAII on B calls and also selects and controls break-out of free FDR, BLR, FDR-X and RR's.
REG Register that receives and records the dial impulses. There is full availability of all registers within a 270 group from stations belonging to that 270 group.
RSA Register finder. Contains a crossbar switch with 10 verticals and 6 horizontals. Connects SNR'S, FFR's to REG.
Directs marker calls and effects break-out within 270 groups. One RSA is required per 270 group.
SGR Relay set containing oscillators for generation of dial tone, ringing tone, busy tone etc. The unit also contains relays for tone interruption and for distribution of tone and ringing signals. One relay set SG R is required per group of 270 stations.
SLA Line finder, final selector. The relay set contains a crossbar switch with 1 0 verticals and 6 horizontals station cutoff relays and control relays.
SLB Selector containing a crossbar switch with 1 0 verticals, 6 horizontals and control relays.
SLC Selector containing a crossbar switch with 1 0 verticals and 6 horizontals. SLC links SLB and SLD in the four stage system (B-call).
SLD Selector containing a crossbar switch with 1 0 verticals and 6 horizontals for connection of FDR'S, SN R B-side, etc. to SLC.
SLD/FFS Selector containing a crossbar switch with 1 0 verticals and 6 horizontals. The switch contains A and B verticals for FFR's and a vertical for BLR.
The SLD/FFS is part of the SLD stage. One SLD/FFS is required per 20 FDR-C.
SLM Marker for identification of calling 90 group and the stations unit digit. One SLM is required per 270 group.
SLMT Marker for identification of the station's tens digit and for indication of the station category. One SLMT is required per group of 90 extensions.
The SLMT relay set also contains the auxiliary connecting circuit HSNR. SNR Connecting circuit. Relay set for connection of a calling station (A) to a register and to the called station (B). SNR's within one 90 group are accessible only to stations belonging to that 90 group. Each 270 group has 5 SNR's common to that 270 group.

Basic facilities

From extension to extension

A call is made by dialling (or keying) the number of the required extension which consists of three or four digits.

If the called extension is free, the calling party will hear ringing tone but if the called extension is engaged then busy tone will be heard by the calling party.

Release on completion of a call can be either first party or both parties.

From extension to operator.

Internal calls to the operator are initiated by dialling (or keying) digit 0.


Classification of extension
The extensions may be classified as follows:

1. Unrestricted extensions which, in addition to calls within the local public telephone area, may also dial their own long-distance (S.T.D.) calls.

2. Trunk-barred extensions which may dial calls within the local public Tele phone area, but have restricted facilities for long distance (S.T.D.) calls.

3. Semi-restricted extensions, which can not make outgoing calls but can receive incoming calls.

4. Fully restricted extensions, which are barred from both incoming and outgoing exchange line calls.

5. Vacant i.e. number not allocated

Categories 1, 3, 4 and 5 are easily arranged by strapping in the exchange. Category 2 requires additional equipment (see Add on facilities).

Outgoing calls
Outgoing exchange line calls are made by dialling a prefix digit and the wanted external number.


All extensions except classes 4 and 5 can hold an incoming or outgoing exchange line call and make an inquiry call to another extension or the operator, afterwards reverting to the original caller.

Depending on the system employed, an inquiry call to an extension is initiated either by: dialling a digit, flashing the switch hook or pressing the service button followed by dialling the required extension number

. In the following description we have assumed that inquiry is initiated by means of a service button.

Switching between the external caller and the extension is done by pressing the button and may be done as often as required provided the inquired extension does not replace. Inquiry calls to the operator are initiated by pressing the service button and dialling 9. Return to the external caller is effected by pressing the service button provided the operator has not answered. The operator controls the external call when answered. If an error is made during dialling of an inquiry call, the call is abandoned and return to the original call is achieved by momentarily replacing. The same procedure applies if all inquiry units are engaged.

Automatic transfer

External calls may be transferred by an extension to another extension or to the operator. For the transfer of a call the procedure is as for an inquiry call and after dialling is completed the call is transferred by the calling extension replacing the handset. Transfer may be effected before or after the called extension answers.

If an extension attempts to transfer a call to an engaged or a barred extension, the call will be re-routed to the originating extension.

Incoming calls are answered at the operator's console except under night switching conditions. Calls are routed by the operator to the required extension.

Revertive calls

Revertive calls are set up by the operator to a public network number and then reverted to the extension.

Announcing (trunk offer)

The operator is able to announce calls to busy extensions. A warning tone is given to indicate that a third party is across the conversation.

Operators' circuit

Each operator position is equipped with one bothway information circuit to provide operator access to and from extensions.

If required, additional operators' lines can be provided on the console in lieu of FDR's 10, 20, 30, 40 and 50.

Automatic operator recall

A recall to the operator occurs after 30-45 seconds under the following conditions:

a. An exchange line call has been extended to an extension and the extension does not answer within the specified time.

b. An exchange line call has been placed on camp on and the extension does not become free.

c. An exchange line call has been parked and is still awaiting further attention.

Serial calls

If a public exchange subscriber wants to speak to several extensions in turn the operator connects the call in series. This implies that the call is returned to the operator at the end of each successive conversation at which stage the operator switches the call to the next required extension.

Key sender
To enable the operator to call an extension.

A call connected by the operator to an engaged extension is switched through automatically when the extension becomes free. Only one call at a time may be camped on any busy extension.

Provided on the operator's position to permit the operator to speak to either party separately or to both simultaneously as required.

Night service
Three types of night service are available:-
1. Individual night service
One exchange line is switched to a predetermined extension.

Exchange lines 1-40 can be switched to any extension in 270-group 1
Exchange lines 41 -100 can be switched to any extension in 270-group 2

2. Common night service
A group of exchange lines are switched to a predetermined extension.
Exchange lines 1-40 can make one or several groups but must be switched to an extension in 270-group 1.
Exchange lines 41-100 can make one or several groups but must be switched to an extension 270-group 2.

Calls answered by such a predetermined extension may be transferred in the normal manner after the required extension has answered.

3. Universal night service (see add-on facilities)
Automatic night service
If the switchboard is left unattended, automatic night service takes place 30-45 seconds after the arrival of an incoming call.

Add on facilities
Tie lines
Each tie line requires a relay set which occupies one exchange line position and one extension position. Tie line calls to another PABX are initiated in the same manner as an outgoing call but with a different access code. On incoming calls via tie lines the extensions are dialled directly by the distant PABX operator or extension user.

If signalling systems other than E and M are employed an SIR relay set is also used.

Group hunting
The exchange can be equipped for calling a group of extensions by a common group number.

Extensions within a group can be called on their individual number,

The extensions may be arranged in 10 groups of 2. Any 20 extension lines within a 270 line group can be combined in groups of 21 4-10, 20 lines.

The hunting is sequential and can be easily changed by simple strapping alterations.

Direct inward dialling
This permits subscribers on the main exchange system to dial directly any open (i.e. non-barred) extension in the PABX without the assistance of the operator.

Extensions receiving such calls will have the same inquiry and transfer facilities as on ordinary incoming calls.

Calls to extensions which do not answer or are busy may be re-routed to the operator.

The in dialling circuits which require a special FDR are one-way only.

Before this facility can be provided the central office must be suitably prepared.

Control of long-distance calls

By adding ITL-ITK equipment it is possible to prevent selected extensions from dialling to the trunk network.

Automatic call back
An extension calling a busy extension can connect to a waiting circuit by dialling a code digit (3) and replacing. As soon as the wanted extension becomes free the waiting circuit will recall the originating and the called extension.

While the originator is waiting for the other extension to become free, he can still use the telephone for other calls. When the calling extension is engaged when the party put on call back becomes free, call back is effected provided this conversation finishes within 20 seconds.

Push button dialling
By the addition of tone receivers and associated equipment calls may be set up by means of pushbuttons instead of dials.

The tones used for pushbutton dialling are in accordance with the CCITT recommendations.

If the public exchange is not prepared for pushbutton dialling number senders must also be provided to convert tones to decadic impulses. When pushbutton dialling is provided inquiry calls must be initiated either by means of the service button (i.e. earth button) or by hook flash.

Both dial and push button instruments can be used on the same installation.

Universal night service
By the addition of N KA relay sets incoming calls during night service sound one or more bells in the premises. The call may be answered from any extension by dialling the answer digit, e.g. 8.

The call may thereafter be transferred to the required extension in the usual manner.

Hotel-Motel features
Single digit dialling
On a hotel-motel PABX an extension user can reach certain PBX extensions (usually service departments) by dialling a single digit code.

Room numbering
The extension number coincides with the hotel room number, within certain limits

Message waiting
The operator can light a lamp on the extension instruments to indicate to the extension user that a message has been left with the operator.

Room number indication
The operator and the service departments can be provided with number displays which display the room number of the calling extension.

Taken from Ericsson publicity leaflet

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Last revised: November 26, 2010