> > Since a Bell modem would cause a lot of problems on an old > > UK phone line, > > Why is that? Are the electrical characteristics of the phone line > actually that different from US lines?
Electrically the lines are almost identical (there are differences in things like ringing voltage and frequency, but you can plug a US telephone into a UK line with no technical problems). The problem is that one of the 300 baud Bell tones is effectively the same as (I think) the long-distance clearing signal. Something like that anyway. If you connect your 300 baud US modem over here it will effectively cause the exchange to hang up. Modems sold in the UK have/had to be incapable of sending the Bell tones, for that reason. One famous one based on the AM7910 chip had a pin in the mode selector switch that prevented it being set to the positions for the Bell tones. This pin could be removed without even opening the case. I did that to mine, as I use it on a line simulator (not the public network) for testing US modems, etc. -tony
