Panel call indicator
PCI, originally designed along with the panel type of telephone office, was intended to allow subscribers in automated exchanges to dial numbers in manual offices the same way they dialled numbers in panel offices. As a format of interoffice signalling, PCI is one of multiple options retained for compatibility in the #5 Crossbar switch, a later system which served as the platform for the initial DTMF push-button telephone services.
For PCI to achieve its purpose, the panel office sent the requested number to a lit display (call indicator) in the manual office. The switchboard operator at a PCI-aware manual office reads the number from the call indicator and completed the call in the usual way.
In the British Director telephone system, Coded-Call Indicator working (CCI) filled a similar role, as it displayed dialled telephone numbers at the local manual exchange in a mixed (automatic and manual) linked-number area.
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 sender 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 lines. 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.
Digits are sent in order 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 bits 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.
|1||2||3||4||5||6||7||8||9, 13, 17||10, 14, 18||11, 15, 19||12, 16, 20|
|Stations (Ten-Thousands)||Thousands||Hundreds, Tens, Units|
|Stations (Party Letters)||4||=||–||4||–||+||–|
The Director telephone exchange was based on a store-and-forward handling of dialled numbers, originally with seven digit numbers. The director would receive all seven digits, use the first three to determine the destination exchange, then send the last four digits onward. For CCI working, the director would wait until the equipment at the destination was ready and then send each digit as a series of current pulses. Much like PCI signalling, each pulse had three possible levels (weak positive current, weak negative current, strong negative current) and a digit consisted of at most four pulses.