Talk:E and M signaling

message: Anyone have comments? Its kinda late for me to be thinkin' clearly tonights. I'll check this tommorow.
--Othtim
Hi Im working on a NOKIA  4 wire E&M card it is different in how it uses the M end 
the M end on a Nokia card requires an earth (the nokia card actually has a negitive battery behind its sensor ) when it sees the earth is triggers the earth at the remote end in the normal manner.
this is called (by Nokia ) "non-inverted e&m signalling"
it might be worth adding somthing to this effect it would have saved me a bit of confusion
cos I was trying to inject a -48v just like I always did
good luck
 Derek
There are variants of E & M, at least 5 types.
These make use of various versions of signal state for the E & M lines.
Depending on which side of the communication link you are on (Side A & Side B) the signal is referenced to GND & -48VDC
More info is available from manufacturers documentation (eg Harris who make Mux/Demux units with E & M interfaces) including pictures etc.
From the CISCO site that discusses E&M the best:
E&M Type I—This is the most common interface in North America. 
Type I uses two leads for supervisor signaling: E, and M.
During inactivity, the E-lead is open and the M-lead is connected to the ground. 
The PBX (that acts as trunk circuit side) connects the M-lead to the battery in order to indicate the off-hook condition. 
The Cisco router/gateway (signaling unit) connects the E-lead to the ground in order to indicate the off-hook condition. 
E&M Type II—Two signaling nodes can be connected back-to-back. 
Type II uses four leads for supervision signaling: E, M, SB, and SG.
During inactivity both the E-lead and M-lead are open. 
The PBX (that acts as trunk circuit side) connects the M-lead to the signal battery (SB) lead connected to the battery of the signaling side in order to indicate the off-hook condition. 
The Cisco router / gateway (signaling unit) connects the E-lead to the signal ground (SG) lead connected to the ground of the trunk circuit side in order to indicate the off-hook condition. 
E&M Type III—This is not commonly used in modern systems. 
Type III uses four leads for supervision signaling: E, M, SB, and SG.
During inactivity, the E-lead is open and the M-lead is set to the ground connected to the SG lead of the signaling side. 
The PBX (that acts as trunk circuit side) disconnects the M-lead from the SG lead and connects it to the SB lead of the signaling side in order to indicate the off-hook condition. 
The Cisco router / gateway (signaling unit) connects the E-lead to the ground in order to indicate the off-hook condition. 
E&M Type IV—This is not supported by Cisco routers / gateways. 
E&M Type V—Type V is symmetrical and allows two signaling nodes to be connected back-to-back. This is the most common interface type used outside of North America. 
Type V uses two leads for supervisor signaling: E, and M.
During inactivity the E-lead and M-lead are open. 
The PBX ( that acts as trunk circuit side) connects the M-lead to the ground in order to indicate the off-hook condition. 
The Cisco router / gateway (signaling unit) connects the E-lead to the ground in order to indicate off-hook condition. 
Audio Implementation (two-wire / four-wire) 
There are two distinct types of audio interface (two-wire or four-wire). These implementations describe the number of wires used in order to transmit audio signals.
With the two-wire implementation, full-duplex audio signals are transmitted over a single pair which consists of tip (T) and ring (R) leads. 
The four-wire implementation provides separate paths to receive and send audio signals which consists of T, R and T1, R1 leads. 
Note: Even though an E&M circuit can be called a four-wire E&M circuit, it is likely to have six to eight physical wires, based on the signaling type and audio implementation used.
This is a good discussion of the various types of E&M signalling. It should replace most of the E&M information in the article.
Ron 03:59, 15 December 2006 (UTC)
thread_title: Comments

message: I had always understood that “E&M” stood for ‘Engineering and Maintenance’; not ‘ear and mouth’.
When a link was being set up the main bandwidth was used for the subscriber’s circuits, but a bit of capacity was taken off the top or bottom for the technicians to use when setting up or maintaining the link. 
Perhaps it’s a British v. American difference?? Percy Cameron (talk) 04:08, 23 September 2008 (UTC)
thread_title: Engineering and Maintenance