Re: Ringer Equivalence Number Simulator

Hello Clyde: The telephone line operates in the "current loop" method of 
signal transmission. In times past "current loops" was the preferred mode 
for transmitting data across distant points due to the basic noise immunity 
inherent to the application. First a few characteristics of the telephone 
line looking at tip and ring terminals.

Telephone on the hook: 50 VDC, no ringing.
Telephone on the hook: 100 VAC at 20 Hz, ringing.
Telephone off the hook: 5 to 6 VDC
Current source: 20 to 120 milliamperes
Load on line off the hook: nominal 600 ohm

The ringer responds to an alternating voltage while the normal direct 
voltage is temporarily removed by a hook-detect trip relay at the central 
station. At one time, the telephone companies enforced the line fee based 
upon how many (ring equivalent) loads were hooked to a single phone line. 
The advent of high impedance ring sensors rendered their technique 
obsolete, and the REN is no longer enforced. In years past, people would 
disconnect the "bells" of additional phones so they could be used on a 
"one"-phone line! I dug further to find-out how to characterize the 
mystical REN load. The following excerpt is the closest I found. 

"REN stand for Ringer Equivalent Number. It is a measurement of how much 
ringing power certain telephone equipment takes. REN numbers are used in 
USA to determine how many telephones you can connect to same telephone line 
and still get them ringing properly (typical line can drive about 3-5 REN 
load).
The definition of 1 REN is the ringer power required by one ringer of an 
AT&T standard 500 series telephone set in single-party configuration 
(ringer placed ACROSS the line)."

I hope this answer suffices your curiosity. MAD.SCI Micro.