Caller ID / Caller Display units - Page 1
Forum section: electronics / general

author: steve30
author_rank: Frequent Contributor
timestamp: November 17, 2014, 04:51:33 pm
content: I've bought a couple of old Caller Display units, with the hope of getting the telephone number out and into either a PC or Arduino etc.In England, the Caller ID is transmitted as FSK data. In these units there isn't a dedicated FSK decoder, or a dedicated Caller ID decoder chip. The first one (BT Call Reminder), pictures of which are attached, has a blob chip which interfaces to the display, an LM324, and some transistors.Could anyone suggest what the LM324 is doing? Is the FSK likely to be decoded in the blob chip, or in the external circuitry?Here's a stock image of the unit:http://ecx.images-amazon.com/images/I/41bErR-t1QL.jpgI also have a BT Caller Display 1000. I'll post pictures of that later once I've had a poke around with the oscilloscope.

author: SeanB
author_rank: Super Contributor
timestamp: November 17, 2014, 05:47:19 pm
content: The LM324 is doing the data extraction and delivering the 1200Baud caller id string to the uart in the blob. There the uart is basically just looking for a valid data stream and decoding it. I would guess the unit simply uses the capacitors to couple the line voltage into the opamp and clamps it to limit the ring signal level, then possibly detects a ring voltage in the one part of the opamp to trigger the MCU, to start the serial decoder to get the data stream, then decode and display before going back to sleep.Very likely the blob has a 8052 core inside with integrated LCD driver, and onboard 8k ROM and 256 bytes of RAM used to store the number and to act as a stack and scratchpad memory. Probably does not have a RTC clock on board, even if it is using a 1.024MHz crystal that looks like a watch crystal.

author: steve30
author_rank: Frequent Contributor
timestamp: November 17, 2014, 07:08:00 pm
content: Thanks Sean. That makes sense. I had a look at the outputs of the LM324 (with my oscilloscope) and couldn't see any data between rings, but I might have missed it. I'll have a poke around and see if I can trace out some of the circuit.

author: baljemmett
author_rank: Unknown
timestamp: November 17, 2014, 08:41:04 pm
content: Quote from: steve30 on November 17, 2014, 07:08:00 pmI think it might actually come down before the first ring - I remember my old caller display unit, similar to yours, would update with the caller's number a fraction of a second before the phone started ringing.

author: AndyC_772
author_rank: Super Contributor
timestamp: November 17, 2014, 08:48:16 pm
content: Caller ID data arrives before the first ring in the UK. I believe it's between the 1st and 2nd rings in the US.

author: steve30
author_rank: Frequent Contributor
timestamp: November 17, 2014, 08:51:38 pm
content: Quote from: baljemmett on November 17, 2014, 08:41:04 pmI think you might be right there. The display can display the data before the first ring. Also, looking with an oscilloscope, there is something which looks like data before the first ring.Attached are the pictures of the BT Caller Display 1000 if anyone is interested.

author: m100
author_rank: Regular Contributor
timestamp: November 18, 2014, 03:10:34 am
content: These docs might be useful. Some UKcable networkprovided lines were (historically?) more like the US signalling 'between rings'http://www.sinet.bt.com/sinet/SINs/pdf/227v3p6.pdfhttp://www.sinet.bt.com/sinet/SINs/pdf/242v2p4.pdf

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 18, 2014, 03:34:20 am
content: Beware that most scopes have the ground connected to actual earth ground. While a phone line doesn't supply enough current to do spectacular damage, grounding points of the circuit would likely stop it from operating correctly.

author: coppice
author_rank: Super Contributor
timestamp: November 18, 2014, 03:48:58 am
content: Quote from: steve30 on November 17, 2014, 04:51:33 pmMost of these caller ID unit have a very crude FSK decoder. They get away with this because the signal comes from the local exchange. That means its a strong signal with a high SNR. With such a signal you can get away with just amplifying the signal to a square wave, and timing the zero crossings in a MCU. The signal quality doesn't have to degrade much for this to fail, but these things had to be cheap and low power.In the US there is a huge silence between rings, and the FSK is sent there. The UK ring pattern doesn't allow that, so they send the FSK before the first ring. To allow the receiver to be a low power battery operated device they send a line voltage reversal before the FSK signal begins. This wakes up the receiver and starts the FSK decoder.

author: steve30
author_rank: Frequent Contributor
timestamp: November 18, 2014, 05:21:41 am
content: Thanks for the replies guys.Having learnt to not short the phone line to ground a couple of years ago, I was doing differential measurements. Although I think this is a case where an isolated digital storage scope would be handy.

author: tinytim
author_rank: Contributor
timestamp: November 19, 2014, 09:00:14 pm
content: hey steve, im in Dorset, can do some teardown photos of a CLID box that i have here, if it looks like it could be hacked, you are most welcome to it.Edited: just took a few pics - no LM anything in this one, its all system on chip

author: MLXXXp
author_rank: Frequent Contributor
timestamp: November 20, 2014, 10:46:07 pm
content: As an alternative to hacking a call display unit, you might want to try using an old analogue computer modem. Many of these could be set to detect and output the caller ID info on the serial port.http://ascendis.com/callerid/help/hyperterminal_test.php

author: steve30
author_rank: Frequent Contributor
timestamp: November 21, 2014, 05:58:48 am
content: Quote from: tinytim on November 19, 2014, 09:00:14 pmThanks for the pictures. Not sure if there'd be much I could do with another blob chip one. It would be cool to get one which has 'real' chips.Quote from: MLXXXp on November 20, 2014, 10:46:07 pmI'm working on this now. Unfortunately my existing modems don't seem to work with UK caller ID, but I've bought another off eBay, so hopefully that will. Second hand serial modems are quite cheap, so I'll probably find one somewhere which supports it.Apparently some cheap USB 56k modems support it. That could work well for say, a Raspberry Pi, but I'll stick with RS232 modems for now.

author: FreddyVictor
author_rank: Regular Contributor
timestamp: October 24, 2015, 10:37:11 am
content: just seen this article (promoted by elektor) for a phone-stopper (italian)http://www.purchiaroni.com/progetti/PubbliStop/PubbliStop.htmltranslation to english:https://translate.google.com/translate?sl=it&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.purchiaroni.com%2Fprogetti%2FPubbliStop%2FPubbliStop.html&edit-text=appears to be able to decode phone numbers so could be useful

author: Andy Watson
author_rank: Super Contributor
timestamp: October 24, 2015, 11:04:31 am
content: Quote from: steve30 on November 21, 2014, 05:58:48 amDo you have to have caller-ID enabled at the exchange? I looked into making a caller-ID device some ten years ago. At that time caller-ID was an extra facility that BT charged for, also, it was mutually exclusive with the use of broadband on the same line - that stopped the project in its tracks.

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2015, 11:48:50 am
content: CLI and ADSL are compatible, as the CLI data is sent as audio, in the bandwidth allocated to the POTS service, as straight serial data in the silence after the first ring. Stripped off by a simple capacitor and clipping diode network, and decoded as it is a 1200baud data stream. that they do not want to add CLI onto ADSL is just laziness on their part, they are services added onto the line by different equipment. The CLI is part of the line card in any case, it is not an extra part, merely a firmware switch to turn on the existing function already there, no extra work required other than updating a single bit in a database.ADSL is a separate service added by routing the subscriber line pair through the frame to go into the ADSL combiner in the DSLAM instead of going past it to the final output frame. Nothing there interferes with the other, they are designed from the get up to work together. I have CLI on lines and ADSL on the same lines, with no issues at all. Only lines you cannot have CLI on are TELEX lines, but there the sending station ID is always printed in any case. I am sure you can run ADSL over the TELEX line as well, ISDN TA also has by default CLI sending ability, either over the analogue POTS connections for each bearer channel or over the digital data link.If BT will not do it then simply cancel the BT internet, turn on CLI on the line and use Virgin to give you ADSL over the same copper pair. As they rarely talk to each other they will never know. If you change back to BTinternet they will not remove the cLI as it is a different division in any case. Try to escalate the query out of the helldesk to somebody who actually knows how the network is designed to operate, not a call centre droid who can only follow a script without thinking at all.

author: steve30
author_rank: Frequent Contributor
timestamp: October 24, 2015, 12:30:41 pm
content: Quote from: Andy Watson on October 24, 2015, 11:04:31 amYes, we pay extra for it. The modems I had handy last year did not support caller ID (not all do), but I bought a US Robotics Courier modem off eBay, and that supports it fine. It will happily spit the caller ID data out to a serial terminal when someone calls. Unfortunately I never got around to writing a programme to make use of it and have just been using the standalone caller display unit on its own.