Follow us on TwitterSSDC5 Private Circuits


CONTENTS OF
TELEPHONES UK

Home
Up
A-Z Index
Acknowledgements
Bells & Tone Callers
Connection Boxes
Dial Labels
Telephone Exchanges
Feedback
History
Kiosks & Payphones
Latest Additions
Line Jacks
Links
LST & House Phones
Miscellaneous
Museums
Network Serv. Codes
Nostalgia/Documents
Old Dialling Codes
Shared Service
Sounds
Speaking Clock
TAMs & Cordless
Technical Information
Telephones Pre-1960
Telephones 1960-80
Telephones Post-1980
Tools & Testers
Vehicles
Wiring Cabinets
Wiring Information

Telecom Systems UK
View our new web site which supplies, installs & maintains Telephone Systems & Door-Entry Systems

Visit Telecom Systems UK - Telephone Engineer on FreeIndex

SSDC5 PRIVATE CIRCUITS

SSDC5 is the abbreviation for Signalling System Direct Current No. 5.  This type of link is intended for the connection by a private circuit of PBXs which are in close proximity to one another. This generally means that they are located on the same site and within a distance of 100 metres. For long distances SSAC15 private circuits are used instead. The link comprises of either 2 or 4 wires for transmission and a further pair for signalling. These are sometimes referred to as 2 wire DC5 and 4 wire DC5.

2 wire DC5 uses the same pair for both transmission and reception and is therefore more susceptible to crosstalk between the signals. 4 wire DC5 uses one pair for transmission of voice and another pair for reception. This reduces the crosstalk or interference, between the incoming and outgoing speech. 

4 Wire DC5

 

A

---------------
Transmit
---------------

Incoming Speech

---------------
Receive
---------------

A
B
B
A

--------------- Receive
---------------

Outgoing Speech

--------------- Transmit
---------------

A

B B
E

---------------

Signalling

--------------- 

M
M

---------------

---------------

E

 

2 Wire DC5

 

A

---------------

2 Way Speech

---------------

A
B ---------------

---------------

B
E

---------------

Signalling

---------------

M
M ---------------

---------------

E


The pairs that carry the speech are denoted as A and B. The signalling pair are denoted E for Ear and M for Mouth. This gives rise to the other common term for this type of link: E&M. Signals are received on the E wire and sent on the M wire. At the destination these wires are crossed over so that the E wire is receiving the signals from the other M wire and vice versa.

The E and M wires are electrically isolated from each other, with signals being sent as the presence or absence of a earth on the wire.

Earth On/Off States

The following table lists the earth on/off states on the E and M wires at different states in a call.

Link
Status
Outgoing Incoming
E M E M
Idle Off Off Off Off
Seize Off On On Off
Dialling Off Pulsed (Dialling) Pulsed (Dialling) Off
Answer On On On On
Forward Clear On Off Off On
Backward Clear Off On On Off

 

[Home]  [Telephones]  [Systems]  [TAMs]  [Kiosks]  [Labels]  [Misc.]  [Links]  [A-Z]