Re: Commodore Modems - C1660 and C1670

From: Jack Mooney (
Date: 2004-01-29 01:15:06

On Wed, 28 Jan 2004 11:08:19 +0100, you wrote:

>On Wednesday 28 January 2004 06:25, William Levak wrote:
>> >Hi All,
>> >
>> >I've received a few of these and before plugging into the Australian
>> >telephone system - and having Telstra issue me a bill for $$$$ damages -
>> >I was wondering how universal these modems were. Was the modem different
>> >for different markets? The ones I have came from the USA.
>> As far as I know, telephones everywhere are very much the same, with some
>> slight timing differences.
>> In North America, the dial pulse is +90 V, and the voice signal is -48 V
>> at 21 to 35 mA.  You should be able to measure these with a volt meter.
>wow 48v ? that sounds a bit much :) afaik its ~60ac for letting the phone
>ring, but its ~12vdc for voice...

That sounds a lot closer. <g> 

IIRC, On-hook loop voltage is 48vdc.  Off-hook loop voltage drops to
the neighborhood of 12vdc.  Ring voltage is 60vac, 20-30Hz.

Dialing (no additional voltage required!) is accomplished by either
interrupting the loop with series of pulses at 10 pulse/second, or
generating "touch-tone" tones. 

When I was stationed in the UK while serving in the US Navy during the
1980's, the British equipment was close enough to US specs that we
used our own phones and switchgear on the BT lines.  I also used a US
made Commodore 1660 and 1670 there to communicate with my Compuserve
account across the "pond". Although the modems would go higher, I had
to stay at 110bps or (usually) lower bps rates because of noisy
overseas lines.  Unfortunately, US and European modem standards
weren't compatible so I couldn't ring up many British BBS'es. 

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