Panel call indicator
Encyclopedia
Panel Call Indicator, or PCI, is a form of signalling
used between two telephone office
s.
PCI was designed along with the panel type
of telephone office. The original purpose of PCI was to allow panel subscribers to dial
numbers in manual office
s the same way they dialed numbers in panel offices.
For PCI to achieve its purpose, the panel office sent the dialed number to a lit display (call indicator) in the manual office. The operator
read the number from the call indicator and completed the call in the usual way.
looked up the type of the called office (panel, manual, etc.) and whether PCI was to be used. Then the sender found a trunk to the called office. Once a trunk had been found and the decision to use PCI had been made, the office code was not needed. So only the line number was sent to the manual office.
However, for two reasons, PCI always sends five-digit line numbers rather than four (and the caller may need to dial eight digits rather than seven).
The panel system was designed to work with manual offices of up to 10,500 lines. Callers dialed the office code followed by the line number within the office. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number. For offices with fewer than 10,000 lines, callers dialed four digits but PCI sends a leading 0.
Also, when the panel office was designed, many people used party line
s. Party line numbers were listed with a J, M, R, or W following the line number. The caller dialed the office code, the line number, and the digit corresponding to the letter. Party letters replace the ten-thousands digit described above. That is, PCI sends the letter followed by the four-digit line number. So party letters and line numbers above 10,000 can't be used together.
from most to least significant. The party letter is sent first, since it replaces the ten-thousands digit.
Generally, each digit is sent as a group of four bit
s having the place values 1, 2, 4, and 5. This could be considered a form of biquinary code. The thousands digit is sent using binary-coded decimal
, but with the 1 bit moved to the end of the group. Except for the 1 bit in the thousands digit, all bits are sent in order from least to most significant. An entire transmission uses twenty bits.
Looking at the transmission another way, it consists of twenty "time slots" which may be filled by DC
pulses. The first and third pulse times in a group of four contain either a positive DC current or no current. The second and fourth pulse times in a group of four contain either a light (high resistance
) negative DC pulse or a heavy (low resistance) negative DC pulse. The pattern of current flow (positive, negative, positive, negative) keeps the receiver synchronized
with the sender. This pattern never varies so it contains no information. The existence of a current (in the first and third pulse times) or the resistance of the current (in the second and fourth times) carries the information.
In this table, + represents a positive pulse, – represents a light negative pulse, and = represents a heavy negative pulse.
Note that the codes for the stations digits or letters 0, 1, W, R, J, and M are the same as the codes for the hundreds/tens/units digits 0, 1, 2, 3, 4, and 5.
Signalling (telecommunications)
In telecommunication, signaling has the following meanings:*the use of signals for controlling communications...
used between two telephone office
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s.
PCI was designed along with the panel type
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...
of telephone office. The original purpose of PCI was to allow panel subscribers to dial
Rotary dial
The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...
numbers in manual office
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...
s the same way they dialed numbers in panel offices.
For PCI to achieve its purpose, the panel office sent the dialed number to a lit display (call indicator) in the manual office. The operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...
read the number from the call indicator and completed the call in the usual way.
What is sent
The usual seven-digit US telephone number consists of a three-digit office code and a four-digit line number within the given office. After a caller in a panel office dialed the office code, the senderSender
A sender was a circuit in a 20th century electromechanical telephone exchange which sent telephone numbers and other information to another exchange. In some American exchange designs, for example 1XB switch the same term was also used to refer to the circuit that received this information...
looked up the type of the called office (panel, manual, etc.) and whether PCI was to be used. Then the sender found a trunk to the called office. Once a trunk had been found and the decision to use PCI had been made, the office code was not needed. So only the line number was sent to the manual office.
However, for two reasons, PCI always sends five-digit line numbers rather than four (and the caller may need to dial eight digits rather than seven).
The panel system was designed to work with manual offices of up to 10,500 lines. Callers dialed the office code followed by the line number within the office. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number. For offices with fewer than 10,000 lines, callers dialed four digits but PCI sends a leading 0.
Also, when the panel office was designed, many people used party line
Party line (telephony)
In twentieth-century telephone systems, a party line is an arrangement in which two or more customers are connected directly to the same local loop. Prior to World War II in the United States, party lines were the primary way residential subscribers acquired local telephone service...
s. Party line numbers were listed with a J, M, R, or W following the line number. The caller dialed the office code, the line number, and the digit corresponding to the letter. Party letters replace the ten-thousands digit described above. That is, PCI sends the letter followed by the four-digit line number. So party letters and line numbers above 10,000 can't be used together.
Encoding
Digits are sent in orderEndianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
from most to least significant. The party letter is sent first, since it replaces the ten-thousands digit.
Generally, each digit is sent as a group of four bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s having the place values 1, 2, 4, and 5. This could be considered a form of biquinary code. The thousands digit is sent using binary-coded decimal
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...
, but with the 1 bit moved to the end of the group. Except for the 1 bit in the thousands digit, all bits are sent in order from least to most significant. An entire transmission uses twenty bits.
Looking at the transmission another way, it consists of twenty "time slots" which may be filled by DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
pulses. The first and third pulse times in a group of four contain either a positive DC current or no current. The second and fourth pulse times in a group of four contain either a light (high resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
) negative DC pulse or a heavy (low resistance) negative DC pulse. The pattern of current flow (positive, negative, positive, negative) keeps the receiver synchronized
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
with the sender. This pattern never varies so it contains no information. The existence of a current (in the first and third pulse times) or the resistance of the current (in the second and fourth times) carries the information.
In this table, + represents a positive pulse, – represents a light negative pulse, and = represents a heavy negative pulse.
Note that the codes for the stations digits or letters 0, 1, W, R, J, and M are the same as the codes for the hundreds/tens/units digits 0, 1, 2, 3, 4, and 5.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9, 13, 17 | 10, 14, 18 | 11, 15, 19 | 12, 16, 20 | |||||
| – | – ! 0 |
– | – ! 0 |
– | – | |||||||||||
| + | – | – ! 1 |
– | = ! 1 |
+ | – | – | |||||||||
! 2 |
+ | – | – ! 2 |
= | – | |||||||||||
| + | – | = ! 3 |
+ | = | – | |||||||||||
| = | – ! 4 |
– | + | – | ||||||||||||
| – | + | – ! 5 |
= | = ! 5 |
– | = | ||||||||||
| – | = ! 6 |
+ | = | – ! 6 |
+ | – | = | |||||||||
| + | = | – ! 7 |
+ | = | = ! 7 |
= | = | |||||||||
! 8 |
– | + | – ! 8 |
+ | = | = | ||||||||||
| = | – ! 9 |
– | + | = ! 9 |
– | + | = | |||||||||