Date: Wed, 18 Mar 1998 06:33:02 -0800 From: Andrew Ingraham To: Multiple recipients of list CHIPDIR-L Subject: RE: MODEM voltage? > I want to make a transmision whit a modem, but I don't know which is his > working voltage If you are talking about telephone line modems, there are several voltages. Which do you mean? There is the DC voltage on the phone line when "on-hook" (hung up), which runs around 48V in the USA (sorry, I don't know if these things differ elsewhere), and drops to typically 10V or less when "off-hook". The AC ringing voltage (to ring the bell on your phone) is roughly around 100V RMS at 20-100Hz. Voltage spikes in the event of a wiring short circuit (power wire falls onto telephone wire) or a lightning strike (not a direct hit, just a thunderstorm in the area) run considerably higher. Anything you connect to a phone line must be able to withstand several hundreds if not thousands of volts for short periods of time. The signal voltages between the modems are considerably less, typically less than 1 volt RMS. There are standards for the transmitting (sending) and receiving levels, measured in dBm's or a similar unit of measure; but I don't have those standards. I think the sending level is around -10dBm or so, which gets attenuated to about -20 to -30dBm at the other end of the phone connection. The signal levels between an external modem and a computer are based on RS232, which I think are between +/-5V and +/-15V or so. Regards, Andy