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 | M: V 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 | | SG: G | M: | | SB: 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 | SG: + | M: X | SB: 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 | | SG: G | M: X | SB: 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 | M: G |