In Reply to: Cordless Internet Design posted by bulan on 09/16/00 at 2:08 PM:
I don't think you need 48v from the phone. I remember getting an old 286 with a 2400 baud modem from a friend. I remembered what the last page of a user's manual from a 14.4 modem...
It said that you can connect two modems directly together with a regular phone cable, you can network. One nd o cord in one modem, other end in other modem, never conected to anything else.
I could the start terminal pograms and "chat" (watching the letters pop up like a typewriter-2400bps).
But there are two problems that I could think of.
1. There may be echos. The microphone on the handset is connected to the transmitter on the handset. The speaker on the handset is connected to the receiver on the handset. The base's transmitter and receiver are connected together to the phone lines.
So some of the sound from the microphone is fed back to the speaker. This is so that the phone doesn't "sound dead". I think that modems are imune from this, because it would be almost imposible to eliminate this feedback. But when you connect the modem,
the handset's receiver output (originally to the speaker) and the transmiter input(originally from the mic) would have to be connected together and this connected to the modem. So the mic-to-phone and phone-to-speaker lines will be connected together in 2 places, one in the base and one in the modified phone.
So it might start to oscillate due to feedback (like putting the microphone by the speakers in a PA system). This can be overcome by bypassing some of the modem's line-interface circuitry. The modem has a ring detecter connected to the phone. There is also a transformer connected to the lines by a relay. On the other side of the transformer, the audio inputs and outputs
are connected. you could probabally do away with the ring detector, relay, and transformer and conencting the handset directly to the modem's audio inputs and outputs, making sure that there is no connections between the two. But I would rather not modify the modem. I want a phone with a phone wire comming from it. maybe it won't oscillate or some combination of reistors, capacitors and amplifiers will do away with it.
2. Will the line quality be reduced and slow the modem speed?
I think that this would work.
I ahve a phone or two I can sacrifice (I have 3 cordlesses). 2 cheap, analog 900MHz phones and a cheap(my friend gave me and I resoldered the crystall!FRee!) analog 46 MHz phone. The black 900 MHz phone is the family phone, and can't be screwed with. The blue 900MHz ha a story to tell...
I got boared one night and had some silicon sealant/glue, so I waterproffed it. It got thrown several times when I got mad(William says i should throw my chair instead of the phone....). When I went swimmin I took it with me...and tested the water proofing...and it failed miserably. But it did keep the water in. After I shook most of it out, it put it under my halogen desk lamp. And I melted it. The glue makes it so that I can't open the phone.
I then take out my 30,000 RPM dremmel with a cutting disc (which shot off and hit me). When got the phone apart, the transmitter didn't workbecause it coroded. Out comes the RV/boat cleaner and the phone works!
That blue 900 mhz also had a handsfree headset thing that did work well for me, so I chopped the cord and used it to make a homemade graphlink (so that I can transfer games from the internet to my TI-86 for use in algebra and tech (the amzing think about tech is that the day's work takes 5 minutes to do, and everyone thinks the same. The robots are used for RobotWars, not moving balls around, the big rulers are used for swordplay, not measuring, the 3 side rulers are called Mr. Perez Rulers, not architect squares,
the T-squares become hammers/battleaxes, the HeNe laser becomes a shoulder-mount-robotron-death-ray, the bechvises smash pennies, and 95% of the movies on the stopframe-animation-with webcams and plastic-dinosars computer are sexualy explicit. There is always time o do my math homework. It's been like this for years and Mr. Perez doesn't care)
sorry for the long story, but the phone (and it's parts) deserves to have it's story told, it's been through so much.
The 49 mhz phone came from a friend. The crystall for the PLL fell off. It was wall mounted. He would bump it with his head and it would fall and the counter and bounce off and fall end over head to the floor.
I'll try the mod on the blue 900 (poor fone) first.
One thing you might try is using a digital phone. and discard the A/D converters. These should be capable of high speed bidirectional transmission. Handset connected to COM1 and modem connected to base station.