Geoff Steckel <gsteckel@vergil.east.sun.com> (06/20/91)
Recently I bought a PP 9600-SA modem, and attempted to use it. No joy -- the best I could do was 4800 BPS. Mysteriously, only the incoming data were incorrect -- the remote system received my transmissions essentially perfectly. After a lot of futzing around, I disconnected a two-line phone with a line powered 'in-use' light. Voila! 9600 BPS V.32 worked with very low error rates. The culprit was a U.S. Tron # PL2125 two-line phone. It isn't a good phone -- I'm not sure how it passed the part 68 drop tests, and it had other problems. Still, it's the ONLY commercial example of a line-powered 'in use' light I've seen. My Panasonic and 'SW Bell' both use batteries to power the lights. Since the request for line-powered 'in use' indicators recurs frequently on at least two of these newsgroups, I'm posting this to suggest that they might not be a good idea. One could have the possibility of adding significant distortion to the phone signal, which for a V.32 modem could cause very bad interactions between outgoing and incoming data. Grossly simplified, one could easily present a nonlinear load on the line. Any phone engineers have any comment? geoff steckel (gwes@wjh12.harvard.EDU) (...!husc6!wjh12!omnivore!gws) Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line. This posting is entirely the author's responsibility.
Nelson Bolyard <nelson@sgi.com> (06/22/91)
gsteckel%Ecd@sun.com (geoff steckel) wrote that his line-powered phone
"in use" light was keeping his modem from being able to receive at
9600 bps. He correctly stated that line-powered lights may be
sufficiently non-linear to affect the signal.
Here is a schematic for a phone line in-use indicator that won't
affect your modem. This circuit is a modification of one published in
the article "Phone Line Busy Indicator", by Robert M. Harkey, Modern
Electronics, November 1988, page 49. In that article, Q1 was
incorrectly shown as an NPN transistor.
I built this using Radio Shack parts and the box is now sitting next
to the phone in my kitchen. It has quite succesfully prevented the
old problem of my spouse picking up the phone while in use by my
modem. Unlike some of the other circuits I've seen posted, this one
presents a balanced load to the phone line, and does not interfere
with high speed modems (e.g. I have no trouble communnicating at 19200
bps with my Telebit T-2500 modem). This circuit draws less than 12
microamps from the phone line when on-hook, and even less when off
hook. It should not cause any problems on your phone line (e.g. it
won't cause any telco equipment to flag you line as leaking/needing
repair).
o-----------------o--------o
| | |
| V LED |
| --- |
| | |
o > |
/E > R5 |
| / > | +
B |< | -------
Green (+) o-----VVVVV----o----| Q1 o --- 3Vdc
R1 | |\ | /C -------
| | \C B |/ ---
> o----VVVVV---o--| Q2 |
Phone line > R3 R4 |\ |
> | >E |
| o |
| | |
Red (-) o-----VVVVV----o--------------------------o--------o
R2
R1 = R2 = 2.2 Megohms Q1 = 2N3906 PNP xistor (276-1604 pkg of 15)
R3 = 330 K ohms Q2 = 2N3904 NPN xistor (276-2016)
R4 = 33 K ohms Battery = 2 AAA cells.
R5 = 100 ohms LED = garden variety red LED
Miscellaneous hardware (Radio shack part numbers):
276-159A pkg of 2 printed circuit boards (great for little projects).
I built this circuit and a "hold" circuit on ONE of these PCBs.
270-230 project box (3.25x2.125x1 inch)
270-398 Double AAA battery holder
23-555 pkg of 2 AAA alkaline batteries
Construction note: when I first built this circuit, the light shone
very dimly. The problem was that the Q1 transistor had a "beta" (Hfe)
lower than its rating and was passing insufficient current to the base
of Q2 to light the lamp. Replacing Q1 with another one from the Radio
Shack package of 15 solved the problem.
Disclaimer: I have no relationship with Radio Shack except as a (usually)
satisfied customer.
Nelson Bolyard nelson@sgi.COM {decwrl,sun}!sgi!whizzer!nelson
Disclaimer: Views expressed herein do not represent the views of my employer.Mike Andrews <mikea@chinet.chi.il.us> (06/22/91)
In article <telecom11.473.2@eecs.nwu.edu> gsteckel%Ecd@sun.com writes: > Recently I bought a PP 9600-SA modem, and attempted to use it. No joy > -- the best I could do was 4800 BPS. Mysteriously, only the incoming > data were incorrect -- the remote system received my transmissions > essentially perfectly. After a lot of futzing around, I disconnected > a two-line phone with a line powered 'in-use' light. Voila! 9600 BPS > V.32 worked with very low error rates. I'm not a "phone engineer" but I did work as a tech support rep at US Robotics. Your experience was one we watched for. The early Panasonic Easa-phone speaker phones were famous for causing line noise and modem data errors. It's the LED connected directly to the the phone line that does it, as you suspected. I guess it turns into a selective filter that attenuates the modem's signal.
david@llustig.palo-alto.ca.us (David Schachter) (06/24/91)
In article <telecom11.478.3@eecs.nwu.edu> nelson@sgi.com (Nelson Bolyard) writes: > This circuit draws less than 12 >microamps from the phone line when > on-hook, and even less when off hook. It should not cause any > problems on your phone line (e.g. it >won't cause any telco equipment > to flag you line as leaking/needing >repair). The tarriff requirement in Canada is one microamp on-hook current draw, I recall. David Schachter internet: david@llustig.palo-alto.ca.us uucp: ...!{decwrl,mips,sgi}!llustig!david