Description: Specifies the E&M Lead Signaling type.
LEGEND: V     = -48V
/\/\/ = detector/resister
X     = normally open relay contact
+     = normally closed relay contact
G     = ground
SB    = Signal Battery
SG    = Signal Ground

typeI -
     Leads:  E (output, relay to ground)
             M (input, referenced to ground)

     The tie line equipment (switch/router/etc) generates the
     E-signal to the PBX type grounding the E-lead.  
     The tie line equipment detects the M-signal by detecting
     current flow to ground.  A common ground must exist 
     between the line equipment and the PBX.

              PBX    |  TIE LINE
                     |
     E:   V--/\/\/---|-----X----G
                     |
     M:   V----X-----|---/\/\/--G

typeII -
     Leads: E (output, relay to SG)
     M (input, referenced to ground)
     SB (feed for M, connected to -48V)
     SG (return for E, galvanically isolated from ground)

     This interface requires no common ground between the
     equipment, thus avoiding ground loop noise problems.
     E-signal is generated toward the PBX by connecting it
     to SG.  M-signal is indicated by the PBX connecting
     it to SB.  While this interface does NOT require a
     common ground, it does have the tendency to inject
     noise into the audio paths since it is asymmetrical
     with respect to current flow between devices.

              PBX    |  TIE LINE
                     |
     E:   V--/\/\/---|-----X----
                     |          |
     SG:  G----------|----------
                     |
     M:    ----X-----|---/\/\/--G
                     |          |
     SB:   ----------|----------V

typeIII -
     Leads: E (output, relay to ground)
     M (input, referenced to ground)
     SB (connected to -48V)
     SG (connected to ground) 

     This interface operates similar to type I with respect
     to the E-signal. The M-signal, however, is indicated by
     the PBX connecting it to SB on assertion, and alternately
     connecting it to SG during inactivity. Unfortunately, a 
     common ground must be shared between the equipment.

              PBX    |  TIE LINE
                     |
     E:   V--/\/\/---|-----X----G
                     |          
     SG:   ----------|----------G
              +      |
     M:    ----------|---/\/\/--G
          X          |
     SB:   ----------|----------V

typeIV -
     Leads: E (output, relay to SG)
     M (input, referenced to -48V)
     SB (feed for M, connected to ground)
     SG (return for E, galvanically isolated from ground)

     This interface is fully symmetrical; it does not require
     a common ground, does not inject noise into audio paths,
     and can be connected back to back by swapping E/SG for 
     M/SB.  E-signal is indicated to the PBX by connecting it
     to SG.  M-signal is provided by the PBX connecting it to
     SB.

              PBX    |  TIE LINE
                     |
     E:   V--/\/\/---|-----X----
                     |          |
     SG:  G----------|----------
                     |
     M:    ----------|---/\/\/--V
          X          |
     SB:   ----------|----------G

typeV -
     Leads: E (output, relay to ground)
            M (input, referenced to -48V)

     Type V line equipment indicates E-signal to the PBX by
     grounding the E-lead. The PBX indicates M-signal by 
     grounding the M-lead.  This interface does require a
     common ground, but does allow back to back connections
     by swapping E for M.  This interface is quasi-symmetric
     in that while the line is up, current flow is more or
     less equal between the PBX and line equipment. But noise
     injection is still a problem.

              PBX    |  TIE LINE
                     |
     E:   V--/\/\/---|-----X----G
                     |          
     M:   G-----X----|---/\/\/--V
                     |