This section of The Phonebooth is dedicated to the various Key Telephone Unit (KTU) cards manufactured by Western Electric for use in the various Key Telephone System (KTS) arrangements. The KTU cards are mounted in Key Service Unit (KSU) panels. These cards are primarily used to provide various features in a KTS for both the 1A2 and ComKey on-customer-premise telephone systems.

These cards are used in a variety of KSU panels. A typical KSU panel is pictured to the right. Most cards could be used in any of the standard KSU panels (513, 514, 515, 597, 598, 620 and 642). For small business installations (and some home installations) the 501, 502 or 551 KSUs are used. These KSUs would only work with a small number of KTU cards (400, 401, 415 and 467). Some KTU cards are designed solely for use with ComKey systems.

Each card served a specific purpose. One 400-series card is required for each outside line the system is connected to. Other cards provide additional features for the system, like music on hold (451 and 498) or toll restriction (471 and 479).

On a typical key telephone set (like the one pictured to the left), one card is required to provide the functions for each button on the set. If the telephone set has a button for each of four Central Office lines, plus a button for a manual intercom, four 400-series cards and one 401-series card would be required in the KSU panel to support these functions. If the customer wants music on hold for the Central Office lines, a 451 or 498 card would also be required.

Listed below are all of the KTUs manufactured by Western Electric for general use. The 402 and 404 diode matrixes are not specifically cards, but are still considered KTUs. We are also aware of additional cards designed for use in KSU panels that appear to be specific to military applications. You can click on some of the descriptions for more information about that particular KTU.

	400	Central Office or PBX Line Circuit
	401	Manual Intercom Line
	402	Diode Matrix for Common Audible Signal Control
	404	Diode Matrix for Common Audible Signal Control
	405	Multiline Exclusion This KTU was replaced by the 428A KTU.
	406	Supplementary Hold Detector This KTU was replaced by the 430A KTU.
	407	Dial Intercom 10-Code Selector
	408	Flutter Generator This KTU was replaced by the 430A KTU.
	412	Auxiliary Lamp Relay
	413	Auxiliary Ringup
	414	Manual Signaling, Ringdown Private Line
	415
	416
	417
	418	Short Range, DC Signaling Private Line
	419	Automatic Signaling, Ringdown Private Line
	420	Dial Intercom Long Line
	421	Power Failure Transfer Circuit, Audible Signal Suppression Circuit or General Purpose Relay
	422	Station Busy Selector
	423	Dial Tone, Busy Tone, and Audible Ringback Tone The 423B KTU was later referred to as the 476A KTU.
	424	Dial Intercom 19-Code Selector
	425	Dial Intercom, Flashing Lamp
	426	Ampl. Band Separation and Limiter Ckt. This KTU requires a 427-type KTU.
	427	Freq. Recog-Trnsl This KTU requires a 426A KTU.
	428	Multiline Exclusion
	429	Supplementary Hold Detector
	430	Flutter Generator
	440	Touch-Tone Adapter The 440A KTU was replaced by the 478B KTU.
	444	Selector Extender
	448	Delayed Transfer Control, Variable Delay Timer
	449	Immediate Transfer Control
	451	Music on Hold This KTU cannot be used with the 400H KTU.
	452	Power Failure Ringing
	454	3-Path Intercom
	455	Tone Ringing Singal Generator
	456	Voice and Tone Alterting Circuit
	457	Paging Amplifier
	460	2-Path Intercom
	461	Manual Signaling, Ringdown Private Line
	467	Low-Voltage Monitor
	470	External Signal
	471	Battery Reversal Toll Restriction
	476	Dial Tone, Busy Tone and Audible Ringback Tone
	478	Touch-tone Adapter
	479	Rotary Dial Toll Restriction
	494	Touch-tone Selector
	498	Music on Hold