🚀 go-pugleaf

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.
>
>
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>|Timothy D. Kuehn					timk@egvideo          |
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-- 
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