Private Exchanges and Small Systems

6.1 PBX, PAX and PABX

A business with many telephones needs two kinds of calls: calls inside the building, and calls to and from the public network. The industry developed three names for the equipment that handles them.

• A PBX (private branch exchange) is an exchange on the customer’s premises with trunks to the public exchange, “a ‘branch’ exchange because it operates in conjunction with and forms a part of a larger exchange” (Miller, 1905; Volume 1). The term covers manual and automatic equipment alike, and the Bell System used it for everything.
• A PAX (private automatic exchange) is designed “primarily for local traffic, within the business concern which it serves” (Smith and Campbell, Automatic Telephony, 1921, ch. VI). It is an automatic intercom, often with extras such as paging, conference calls, watchman’s tours and fire alarm, and it has no public trunks at all. Smith and Campbell report that installing a P.A.X. typically raised a firm’s internal telephone use “three to five times.” Automatic Electric advertised its P.A.X. in the New York Times as early as 18 June 1918.
• A PABX is an automatic PBX: dial service inside, plus trunks to the public exchange. The term was used mainly by Automatic Electric and the independents.

The system this project is building, three telephones that can call each other with no outside line, is in these terms a PAX. It is also a very small one.

6.2 The Bell System’s PBX Families

The Bell Laboratories switching history tabulates the Bell System’s PBXs from manual cord boards to electronic systems (Switching Technology (1925–1975), ch. 13). The step-by-step and early common-control families:

Table 1 — The Bell Laboratories switching history tabulates the Bell System's PBXs from manual cord boards to electronic systems ([Switching Technology (1925–1975), ch. 13](https://archive.org/details/bell-system-science-history)). The step-by-step and early common-control families

System	Type	Capacity	Introduced	Notes
740A	step-by-step, with a cordless attendant’s board	88 lines	1926	740B/C (38 lines) 1928; 740E (1950) factory-packaged, to about 300 lines
701A	step-by-step, with a cord attendant’s board	3,200 lines	1929	recoded 701B in 1959; still “the system by which all others were judged” in 1971
711A	step-by-step “satellite”	10,000 lines	—
750A	relay logic	15 lines	1929
755	the first crossbar PBX	20 lines, 4 trunks	1938	built for more than 25 years
555	manual cord board	120 lines, 15 cords	1948	the manual PBX of Volume 1
756A / 757A	crossbar, marker and register	40–60 / 200 lines	1956 / 1963
770A	crossbar	to 400 lines	1970s	the last electromechanical design cycle
No. 101 ESS	stored-program, PAM time-division	3,200 lines per control unit	1963

During the Second World War the Pentagon’s PBX grew to 16,000 lines.

6.3 Automatic Electric’s P-A-X Range in 1929

Automatic Electric’s September 1929 bulletin Strowger P-A-X (Bulletin 1026) lays out its whole small-system range, and it shows how a step-by-step system scales down (bitsavers). It also reports that more than 2,000 organisations were using Strowger P-A-X, and that AE’s Chicago works could produce equipment for 300,000 lines a year.

Table 2 — Automatic Electric's P-A-X Range in 1929

AE P-A-X (1929)	Capacity	How it switches	Power
Type 1	90 telephones, one conversation (“seldom used for more than twenty or thirty”)	a single standard automatic switch; every telephone on one common pair plus its own ringing wire; two-digit numbers	22 V, 11-cell 12 Ah battery, trickle-charged by an electrolytic rectifier
Type 10	10 lines, 2 calls at once; 2 telephones per line, code ringing	simple rotary switches and relays; dial one digit, then “8” to ring	11-cell 12 Ah battery
Type 2	25 lines, 3 calls at once	3 finder switches + 3 connectors, one finder per connector; vibrating-reed ringing converter; 2-digit numbers; automatic intermittent ringing; busy tone	48 V, 23-cell 12 Ah battery
Type 5	up to 100 lines	line switches (one per line, 25 per group, 4 groups) feeding connectors: “seven connectors are usually furnished, although there is space for as many as 20”; 2-digit numbers 00–99	48 V, two 23-cell 24 Ah batteries
Type 7	”unlimited"	"an expansion of the Type 5, with selector switches added to select a connector in the called hundred-line group”	48 V

Later Automatic Electric literature in the Reference Library continues the line: the Type 32A1 P-A-X instructions of 1935; Bulletin 818 (1959) on PAXs, with a PABX appendix; the Type 50 PAX/PABX (Technical Bulletin 322, 1956); the Leich Type 40 and 80 PABX (TB 500-407, 1964); the AE/Leich 40-60D packaged PAX of 1963 (40 lines, expandable to 60, six simultaneous conversations); and AE’s general maintenance of PAXs (TB 565, 1961).

6.4 The Smallest Real Exchange: a 10-Line PAX

The most useful single document for a builder is Automatic Electric’s circuit explanation E-75159, “10-Line Isolated P.A.X. Circuit, 24 Volts,” issue 1 of 29 January 1944, which describes a complete exchange for ten telephones in eleven pages. It is in the Reference Library. Its general description reads like a summary of everything in Volumes 4 and 5, shrunk to the smallest working size:

• Each line has a line-and-cutoff relay (A3). “When a call is initiated, relay A3 operates partially to mark the position of the calling line on the finder switch bank multiple,” and a common start relay operates “to provide for starting an idle finder.”
• Two finder switches, each with a selector relay “so arranged that when both finder switches are idle, the first finder is selected and the second finder switch cannot be started while the first switch is hunting.” That is a distributor’s job done by two relays.
• The finder is a motor-magnet stepping switch, interrupted by relay A1 “to cause it to hunt for the calling line. This hunting action stops when the wipers encounter the bank contact marked by relay A3 of the calling line.” A3 then operates fully “to disconnect itself … from the line.”
• Relays B1 and D1 cut the line through, return dial tone through a capacitor, and ground the line’s busy lead “to make the calling line busy at the connector bank multiples.”
• Dialling is a single digit. Relay A1 pulses the connector switch A4, another motor-magnet stepping switch, “in multiple with relay C1,” which stays operated during pulsing “to keep the impulsing circuit closed when relay G1 operates if the connector switch wipers pass over a busy line.” (E-75159’s letters are its own: its C1 does roughly the job of the E relay in Volume 5, and G1 is its busy relay.)
• Busy test: “If the dialed line is busy ground from the CN lead is encountered … Relay G1 will then operate to return busy tone. … If the dialed line is free, battery from the CN lead is encountered … relay H1 will operate after relay C1 restores.” H1 starts the ringing.
• Ringing comes from a relay-driven ringing converter (K5) switching direct current “alternately and in opposite direction” through a ringing transformer. A chain of interrupter relays sets the ringing cadence for automatic ringing (“X” wiring), or the caller rings by dial control (“Y” wiring). A tone generator relay makes the tones.
• Its remaining sections cover ring-back tone, ring cut-off and switch-through, transmission, and restoring.

E-75159 shows that the complete logic of a step-by-step exchange (line marking, finder start and allotment, hunting, cut-off, dial tone, busy test, ringing, ring trip, release) fits into ten lines, two links and a single digit, on a 24-volt supply with no rotating machinery at all.

6.5 Scaling Down: What a Three-Telephone System Needs

Taking the documented patterns together (the 1929 Type 2, the 1944 10-line PAX, and the hundred-line arrangement of Smith and Campbell), a three-telephone step-by-step system needs:

Table 3 — Taking the documented patterns together (the 1929 Type 2, the 1944 10-line PAX, and the hundred-line arrangement of Smith and Campbell), a three-telephone step-by-step system needs

Function	Minimum	Notes
Line circuits	3 line-and-cutoff relays (or L + CO pairs)	one per telephone
Finding the caller	1 line finder, or 3 rotary line switches	a 100-point finder uses one level; a 200-point finder only its lower bank
Connecting to the called line	1 connector	numbers on one connector level, e.g. 11, 12, 13 (level 1, contacts 1–3)
Simultaneous calls	1 per finder–connector link	with three telephones, one call is the most that can usefully be up at once (a second call would need a fourth party)
Choosing a finder	nothing for one link; a distributor, allotter or start chain for two or more	E-75159 does it with two relays
Power	48 V DC (or 24 V for a 24-V design), ringing, tones	see Volume 8

With one link, two of the three telephones can talk while the third gets no dial tone; nobody is free to call it. That is complete step-by-step behaviour on the smallest possible scale. A second link lets a demonstration show the distributor or allotter at work: one finder in use, the next already assigned. A first selector in front of the connector (the step from AE’s Type 5 to its Type 7) would add the hunting motion that a connector never makes, and a third digit to the numbers.

6.6 A Note on “Connector Expanders”

No document consulted (Automatic Electric’s bulletins, the USAF course, Kerr’s papers, Smith and Campbell, or the US Army’s TM 11-2111) uses the phrase connector expander. Several real things expand what a connector reaches, and any of them could carry the name in a collector’s shorthand:

1. a 200-point connector, which serves 200 lines from two banks, with a relay choosing the upper or lower bank on an extra digit (USAF course, Vol. 3);
2. a trunk-hunting, rotary-hunting or level-hunting (“PBX”) connector, which lets one listed number reach a group of lines (Volume 4);
3. a selector stage placed in front of a group of connectors, as in AE’s Type 7 P-A-X, “an expansion of the Type 5”;
4. more connectors added to a hundred-line group, the connector multiple (“space for as many as 20”).

A nameplate, drawing number or catalogue page on the hardware itself would settle which, if any, it is.

6.7 In the Reference Library

• AE E-75159, 10-Line Isolated P.A.X. Circuit, 24 Volts (1944)
• AE Bulletin 1026, Strowger P-A-X (1929)
• AE Bulletin 32A1, Type 32A1 P-A-X instructions (1935)
• AE Training Bulletin 818, P-A-Xs with P-A-B-X appendix (1959)
• AE TB 322, P-A-X/PABX switchboard Type 50 (1956)
• AE TB 500-407, Leich Type 40 and 80 PABX (1964)
• AE TB 565, General Maintenance of P-A-Xs (1961)
• “An Electromechanical Telephone Exchange”, PoC||GTFO 21:08 (2021)
• W. Aitken, Automatic Telephone Systems, Vol. 2 (private and branch exchanges)

Sources

• A. B. Smith and W. L. Campbell, Automatic Telephony, 2nd ed., 1921, ch. VI. https://archive.org/details/automatictelepho00smit
• Automatic Electric, Strowger P-A-X, Bulletin 1026, 1 September 1929. http://bitsavers.org/communications/automaticElectric/Strowger_P-A-X_Bulletin_1026_Sep1929.pdf
• Automatic Electric, circuit explanation E-75159, 10-Line Isolated P.A.X. Circuit, 24 Volts, issue 1, 29 January 1944 (Reference Library).
• A. E. Joel Jr. et al., Switching Technology (1925–1975), ch. 13. https://archive.org/details/bell-system-science-history
• Automatic Electric bulletins 32A1 (1935), 818 (1959), TB 322 (1956), TB 500-407 (1964), TB 565 (1961) (Reference Library).
• USAF CDC 36251, Vol. 3, ERIC ED194781. https://archive.org/details/ERIC_ED194781