timk@egvideo.UUCP (Tim Kuehn) (01/28/89)
In article <2178@van-bc.UUCP> sl@van-bc.UUCP (pri=-10 Stuart Lynne) writes: >In article <7139@pyr.gatech.EDU> byron@pyr.UUCP (Byron A Jeff) writes: >>-attempting to build a DTMF (Touch-Tone) --> RS-232 converter. I've seen the > >>I helped build a DTMF decoder for a school project last quarter. I also >>-SSI-204 IC's. Who makes these (Samsung?), and what is the difference between > >For example the device should go offhook when "DTR" is raised. A dial tone is >produced when RTS goes high and DTMF recogition starts. What is the definition of an "on-hook" and "off-hook" line? I'm looking for some information on this and other issues about hooking up to Ma Bell and this would be quite helpful. > [some stuff on what would be nice to do with this equipment] >Stuart.Lynne@wimsey.bc.ca {ubc-cs,uunet}!van-bc!sl Vancouver,BC,604-937-7532 I'm working on a project much similar to that for fax machines to mointor and control useage to/from a customer site. This question about DTMF-RS232 has been quite informative for part of my project, but I still need to figure out a way to detect on-hook and off-hook conditions. Also some way of dealing with the 'ring' signal too. Can someone out there supply me with information on how this is done? Or point me towards a publication that has the specs for the telephone lines and what requirements there are for hooking 3rd party equipment up to it? Many thanks for any and all help! +-----------------------------------------------------------------------------+ |Timothy D. Kuehn timk@egvideo | |TDK Consulting Services !watmath!egvideo!timk | |871 Victoria St. North, Suite 217A | |Kitchener, Ontario, Canada N2B 3S4 (519)-741-3623 | +-----------------------------------------------------------------------------+
byron@pyr.gatech.EDU (Byron A Jeff) (01/30/89)
In article <1905@egvideo.UUCP> timk@egvideo.UUCP (Tim Kuehn) writes: >In article <2178@van-bc.UUCP> sl@van-bc.UUCP (pri=-10 Stuart Lynne) writes: >>In article <7139@pyr.gatech.EDU> byron@pyr.UUCP (Byron A Jeff) writes: >> >>For example the device should go offhook when "DTR" is raised. A dial tone is >>produced when RTS goes high and DTMF recogition starts. > >What is the definition of an "on-hook" and "off-hook" line? >I'm working on a project much similar to that for fax machines to mointor >and control useage to/from a customer site. This question about DTMF-RS232 >has been quite informative for part of my project, but I still need to figure >out a way to detect on-hook and off-hook conditions. Also some way of dealing >with the 'ring' signal too. Can someone out there supply me with information >on how this is done? Or point me towards a publication that has the specs >for the telephone lines and what requirements there are for hooking 3rd party >equipment up to it? I'll take a stab at it. Since it looks like you're doing a real application you need a device interface to the phone. It's call a DAA phone line interface (DAA stands for Direct Access Arrangement). What it does is isolates your board from the telephone line and vice-versa. In addition most provide TTL level signals for on-hook, off-hook ring detection etc. An FCC approved DAA is required for real devices. One quoted source of DAA's is Cermatek Microelectronics, Sunnyvale Ca. But the theory for the rest of us hobby hackers might still be useful to know. The concept is fairly simple. On hook just means that the tip and ring lines of the phone are disconnected and only a small current flows between the two lines. Off hook is accomplished by connecting the lines together. The switching circuit at the central office detects that and connects the phone and puts out a dial tone (or connects you to the phone calling you). To put a signal on the phone line you should impedance match to 600 ohms. Many transformer manufacturers make 1:1 600 ohm transformers expressly for this purpose. Ring Detection: The central office will send a 80-120 Volts RMS signal over the phone lines to indicate a ring at a particular frequency. A chip such as the TI TCM 1520A across the lines will detect such a signal and if the output is optoisolated a nice TTL level signal can be created quite easily. Three sources I can quickly point to for help. 1) The ubiquitous and prolific Steve Ciarcia (formerly of Byte but now producing his own hobby projects mag) has had several projects on telephone interface projects. I can't quote particlar issues but these include a customized ring circuit and a single chip modem. 2) A copy of "Understanding Telephone Electronics" from your local Radio Shack. I don't have a copy but I've heard that it's a pretty good reference manual. 3) Data books and application notes. Manufacturers include Silicon Systems Inc (This is where I got a lot of the above info), Motorola Telecommunications Division, National Semi, TelTone, TI among others. They have everything from dialers to detectors to switchers to CODECs. > > >> [some stuff on what would be nice to do with this equipment] > >>Stuart.Lynne@wimsey.bc.ca {ubc-cs,uunet}!van-bc!sl Vancouver,BC,604-937-7532 > > >Many thanks for any and all help! Sure enough. > > >+-----------------------------------------------------------------------------+ >|Timothy D. Kuehn timk@egvideo | >|TDK Consulting Services !watmath!egvideo!timk | >|871 Victoria St. North, Suite 217A | >|Kitchener, Ontario, Canada N2B 3S4 (519)-741-3623 | >+-----------------------------------------------------------------------------+ -- Another random extraction from the mental bit stream of... Byron A. Jeff Georgia Tech, Atlanta GA 30332 Internet: byron@pyr.gatech.edu uucp: ...!gatech!pyr!byron
gbell@pnet12.cts.com (Greg Bell) (01/30/89)
I'm sure you're going to get a hundred responses to that message, but
maybe I can give you some info that nobody else will...
The phone line sits at about 50vdc when all phones are on hook.
When a phone goes off hook, this voltage drops to about 10 vdc.
See, you really don't need any precise specs for this since the
voltage variation is so large. I have a project in front of me that happens
to include an on/off hook detection circuit. I did it with a simple voltage
divider (a 100K and a 4.7K resistor) a zener diode to clamp the ring voltage,
and a transistor (with a pull-up resistor at the collector). When the line
is at 50vdc, the transistor is on, and pulls the output to ground. When the
line is at 8vdc (about), the transistor turns off and the output goes high due
to the pullup resistor.
If you need more help, or a text diagram would help, let me know.
Greg Bell_________________________________________________________
Hardware hacker |
Electronics hobbyist | UUCP: uunet!serene!pnet12!gbell
EE major at UC San Diego |