Here is one for you phone buffs. This may be old stuff to the experts but I needed to do modem to modem contacts over a phone line and made up something to do the job. Not knowing anything about phone lines I scoped a line and worked out what was needed in the way of signals so the modems thought they were connected to an exchange. This was back in 1998. I then bread boarded a set up which you will see in the following PDF story. I dug the simulator out of storage last week and took some pictures and a MOV file.
Cheers, Fred.

Workshop Telephone Exchange Simulator

Seriously cool

Finally, YouTube is working. Video uploaded.

Good work Fred. It obviously worked/works but I would make a couple of suggestions for anybody that is considering making a similar unit to test or demonstrate old phones.

First the 'line' voltage does not have to be 48/50V. Early CB/PAX units operated from 22V to 36V. I have made a similar unit with a 24V RMS 150mA transformer as the power source. Using a simple bridge rectifier arrangement and smoothing capacitor will give about 33V DC which works fine with lines up to about 200R loop resistance. I found that a basic Pi filter made up from two 1000 - 1500μF (100VW) capacitors with a 22R 5W resistor between them (plus a 10K 1W resistor across the output) gives a quiet power supply to the transmission bridge. The four feed resistors can then be reduced to 470R 5W from the 560R you used.

The other thing is that there should only be only two 2 - 2.2μF capacitors linking either side of the transmission bridge, not two pairs of two 2μF capacitors in series. Feed the dial tone via separate 0.1μF capacitors.

Andrew

Hi Andrew, I finally sat down with the simulator and traced the wiring out. I measured some voltages and took some waveshapes. You are right there are some redundant capacitors in there when you look at it. There are also some coupling caps in the dial tone circuit that may be too small in value. I got away with it because the modems have little loading on the ciruits and were very tolerant of voltage and signal levels. I just threw it together and once it worked did not tweak it up at all.

Fred.

Exchange Simulator Circuit Diagram

Thanks Fred. I certainly understand the "its works OK now so don't try to improve it" concept.

I will be interested to see the voltage data and wave forms. I would also be very interested to see the circuit and if possible component values for the Dial and Ring tone generators. My unit was very simple compared to your design and I just used a UJT 'oscillator' to make a 133Hz (approximately) buzz as the dial tone.

It’s unlikely to be the intention of the project, but would be interesting to see how it went with a couple of 2400 baud dialup modems.

Hi Art, the original baud rate that the house alarms I was working with at Jaycar (AvGad) was 300 and I had to set the handshake program for that. Floating around on the web will probably still be found my "avgad notes" by Fred Lever detailing modem to modem contact in the text somewhere. Seeing that was back in decade 1998/2008 I can remember less now than what I knew then, and that was not much as you can gather by the lash up! Do modems ever get used now? All this internet stuff seems to work by magic of some sort and dialling up phones and modems cant exist any more?
Cheers, Fred.

Most modems should still be ok. The only thing that won't work is using a pulse-dialling modem on NBN. You'll have to dial the number yourself then enable the modem to start communicating with the other end.

All this internet stuff seems to work by magic of some sort and dialling up phones and modems cant exist any more?

They do in older commercial installations. Where I work we were still using a 56k modem on the PABX for remote access from the service provider. That PABX was replaced just over a year ago and the modem went with it. A tunnel through the firm's main Internet connections does the remote access thing now, which is the modern way.

Very few ISPs would be offering dialup these days. If one doesn't have cable, fibre or ADSL they'd be using fixed wireless, 4G, or satellite, I'd imagine.

I tried iSelect to get an idea of who is left in the dialup game, just out of curiosity but they don't display search results on their website - they expect to e-mail them to you, along with periodic spam by the looks of things so I didn't bother completing the search.

Hi Fred, and all!
I should have just called myself Brek here to begin with.

I learned that a lot of dial up modems such as US Robotics can be directly connected to each other via a phone cable, and dial into each other as if connected over a phone line.
Since I have a couple of Commodore Amigas set up, this is my plan. The only practical reason to do this instead of using a serial null modem cable that I can see is that you should get a lot more distance out of it between the two computers.

For me I’d just like to hear the old dial up negotiation again, and could actually fire up a BBS program and terminal program.

Hey Relay,
It’s interesting you sent me the PMG stuff, probably at some expense.
This is the end of my first week of full time study, and I should end up an engineer.
Maybe I was supposed to be a PMG or door to door radio tech, but unfortunately, Seek hasn’t yielded too many results for those positions.
My greatest lament is this also explains why I hardly have a hobby any longer.
It’s like taking some steps backward to take greater leaps forward in future. Oh well!