bill@wrangler.WLK.COM (Bill Kennedy) (06/18/91)
Perhaps this question should be directed to Telebit tech support but
I'm not only curious about the answer, but also interested in learning
the significance of what I'm doing. First off brief history. You may
have read my bleatings about trying to get a T-1600 and Trailblazer
Plus working with a cellular phone. Each will work somewhat at 2400bps,
the TB+ better than the T-1600. A PEP connection can't be negotiated or
made with what I have tried so far. Telebit tech support suggested that
I try and lower the signal level (cellular seems to have higher audio
than POTS) and further suggested that I might monkey with the output
power level using the resistor values in TFM. I went hog wild today and
bought 16 1% resistors, each the values called out in T-1600 and TB+ FM
and I'm ready to start plugging them in.
Here's my question. Page E-1 of the T-1600 FM (I'm going to start with
the T-1600 because it's already plugged into everything) starts its table
with a power limit of 0.0 dBm, resistor value of 6.65K. The TB+ FM, same
page also starts at 0.0dBm, different value. More important the TB+
manual says:
"Note that the modem is shipped with a 12.1K resistor installed for a dBm
level of -9.0. In order to conform to FCC requirements, this resistor
must not be changed when connecting the modem to a switched telephone network
in the U.S."
The T-1600 manual is silent with regard to the resistor value required for
connecting to a switched telephone network but it does call out an 18.7K
resistor for a power limit of -9dBm. Raise the hood on the modem and there
appears to be an 18.7K resistor at R34, it measures 18.73K on my meter.
Hmm, I seem to have promised a question a few paragraphs early... It would
seem to me that the T-1600 is set just like the TB+ in that they both have
resistors for a -9 dBm power limit. The Telebit tables range from 0dBm to
-15dBm and I have the right resistors to cover that range. The problem I am
trying to solve is that the modem on this end is pushing too much energy into
the line, I was able to significantly improve my S/N ratio by turning the
cellular volume down as far as it would go. So which way do I go? Do I
progress from -9dBm towards 0 dBm or towards -15dBm? I'm not connecting to
"a switched telephone network in the U.S", I'm plugging into an adapter to
the cellular phone. I'm pretty sure I can't screw up anything but my phone
connection and that's precisely what I'm trying to fix. I'm (as if you
couldn't tell :-) in need of some serious enlightenment here.
As an aside, this cellular modem connection is actually starting to work.
The plan I'm on waives air time after 2000 and before 0700 weekdays and all
day weekends and holidays. My news feed is a landline long distance call
but within my cellular local dialing area (in case someone was trying to
figure out why I was messing with modems and cellular). After I have
conquered the problems I'll post a summary of what to do and what not to
do. I could start the latter right now :-) Thanks for your help,
--
Bill Kennedy uucp {att,cs.utexas.edu,pyramid!daver}!ssbn.wlk.com!bill
internet bill@ssbn.WLK.COM or ssbn!bill@attmail.COMgandrews@netcom.COM (Greg Andrews) (06/18/91)
In article <873@wrangler.WLK.COM> bill@wrangler.WLK.COM (Bill Kennedy) writes: > >Here's my question. Page E-1 of the T-1600 FM (I'm going to start with >the T-1600 because it's already plugged into everything) starts its table >with a power limit of 0.0 dBm, resistor value of 6.65K. The TB+ FM, same >page also starts at 0.0dBm, different value. > Yes, the modems have different circuitry driving the telephone line, so they use a different value resistor to achieve the same output power. > > [....] >I was able to significantly improve my S/N ratio by turning the >cellular volume down as far as it would go. So which way do I go? Do I >progress from -9dBm towards 0 dBm or towards -15dBm? > They're negative numbers, so the larger the number, the softer the signal. 0 dBm is the loudest, -15 dBm is the softest. You want to go softer, so you want the resistor values associated with -10 dBm, -12 dBm, etc. -- .------------------------------------------------------------------------. | Greg Andrews | UUCP: {apple,amdahl,claris}!netcom!gandrews | | | Internet: gandrews@netcom.COM | `------------------------------------------------------------------------'
tad@ssc.UUCP (Tad Cook) (06/24/91)
In article <873@wrangler.WLK.COM>, bill@wrangler.WLK.COM (Bill Kennedy) writes: > > Hmm, I seem to have promised a question a few paragraphs early... It would > seem to me that the T-1600 is set just like the TB+ in that they both have > resistors for a -9 dBm power limit. The Telebit tables range from 0dBm to > -15dBm and I have the right resistors to cover that range. The problem I am > trying to solve is that the modem on this end is pushing too much energy into > the line, I was able to significantly improve my S/N ratio by turning the > cellular volume down as far as it would go. So which way do I go? Do I > progress from -9dBm towards 0 dBm or towards -15dBm? If the output level is now -9 dbm, then going to -11 dbm or -12 dbm would be a lower level. The actual voltage output is referenced to 1 mw on a 600 ohm load, which is about 775 mv (.775 volts) for 0 dbm. For -9 dbm, it is about 275 mv. -12 dbm is about 195 mv. -15 dbm is about 138 mv. Tad Cook Seattle, WA Packet: KT7H @ N7DUO.WA.USA.NA Phone: 206/527-4089 MCI Mail: 3288544 Telex: 6503288544 MCI UW USENET:...uw-beaver!sumax!amc-gw!ssc!tad or, tad@ssc.UUCP or, kt7h@polari.uucp or, 3288544@mcimail.com