Cabletron Systems 1800 E&M Grounding Considerations

C-16 SmartSwi tch 18 00 4.0 U ser Gui de, Rev 01

The Type II interface requires no common gr ound; instead, each of the two signals has

its own return. For the "E" signal, the tie line equipment permits current to flow from

the PBX; the current returns to the PBX's ground reference. Sim ilarly, the PBX closes

a path for current to generate the "M" signal to the tie line equipment.

A variation of Type II, Type III uses the SG lead to provide common ground. With

this configuration, the PBX drops the "M" signal by grounding it, rather than by

opening a current loop.

Type IV is symmetr ic and req uir es no common g roun d. Each sid e closes a cu rrent lo op

to signal; the flow of current is detected via a resist ive load to indicat e the presence o f

the signal.

Type V is a simplified version of Type IV. This is also a symmetric interface, using

only two wires. Type V requires a common ground between the PBX and the tie line

equipment; this is provided via the SG leads.

E&M Grounding Considerations

Proper grounding is essential for correct operation of telephone equipment in E&M

applications. E&M signaling works by applying DC voltages to the "E" and M leads

in the process of connecting and disconnecting. Without a proper ground, the DC

voltages do not have a common point of reference, which can result in unintended or

sporadic signaling. To avoid grounding problems, certain conditions must be con-

sidered.

First, check the distance between the PBX and the SmartSwitch 1800. If the two are

not within close p roximity, then their AC grounds may n ot be the s ame. If the S mart-

Switch and the PBX are in the same room, however, it is likely that they share the

same AC ground.

Second, in an E&M application, the common ground must be the telco/PTT ground.

This assures that the ground potential of the PBX telephone interface and the Smart-

Switch telephone interface are the same.

Based on the above conditions, grounding may be established directly or indirectly.

The direct method is to connect a grou nding wir e between the telco /PTT inter faces on

the PBX and SmartSwitch. You can do this by connecting the ground wire from the

PBX to pin 8 (SG) of the RJ45 connector on the SmartSwitch. This will establish the

same ground potential at both interfaces.

For va rious reason s, di rect gr oundi ng is not a lways possib le or pract ical. To es tablish

an indirect ground, make s ure the telco/PTT groun ds of the PBX and the SmartSwit ch

are connected to their respective AC grounds. The AC grounds of the two devices

must be at the same potential as is usually true when both devices are located in close