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Old 12th Oct 2019, 8:30 am   #6
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 3,948
Default Re: Dial-up bulletin boards

I test modems by connecting them to another known-good modem via a telephone line simulator.

For the known-good modem I like the old Miracle WS2000 for 300 and 1200/75 baud use. It can do just about anythingthe AM7910 chip is capable of, set by a rotary swtich on the panel.

For the line simulator there are basically 3 options :

1) Make one, there are circuits on the web and in books with various features

2) Buy a simple one, I am sure they are still listed on Ebay

When I was looking into this about 7 years ago those 2 options would have cost about the same, the advantage of (1) is you have documentation and can repair it, the advantage of (2) is you don't have to make it.

And then there's

3) Buy a high-end telephone line simulator which will generate the right dial and ring tones, ring timing, let you degrade the signal to similate a faulty line and so on. These can be very expensive even second-hand...

I was lucky, I found a non-working line simulator for about the same cost as making a simple one. It's a fearsome unit, a few hundred ICs inside (including 6 microprocessors one of which is a DSP) over 10 circuit boards. Fortunately I found the faulty LM311 comparators and the voltage regulator that supplies them quite quickly and it now works fine...
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