home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
The X-Philes (2nd Revision)
/
The X-Philes Number 1 (1995).iso
/
xphiles
/
hp95
/
modem.doc
< prev
next >
Wrap
Text File
|
1995-03-31
|
4KB
|
92 lines
From Usenet:
17 Sep 91 17:09 Steve Harper: Beware WorldPort?
Unfortunately, providing a modem solution for the HP 95LX
has not been as simple as we had hoped. Here is a repeat
of my posting of several months ago as well as some new
information:
Most portable modems do not have an on/off switch; rather
they rely upon the DTR line from the terminal device to
turn them on or off. Normally, DTR will be low when the
serial port is unused (modem off) and will go high when
the serial comm program is run (modem on). This conserves
the modem battery (a precious resource) as much as possible.
For various reasons (power, size, cost), the HP 95 only has
a "three-wire" (TX, RX, GND) serial interface. There is no
DTR line to turn the modem on. One way to force the modem
to turn on is to use the modem ac adapter. ANY OF THE
WORLDPORT MODEMS WILL WORK WITH THE HP 95 PROVIDED THE
MODEM IS ON AC POWER. This work-around is hardly a
complete solution, however.
Fortunately, the Worldport modems have a mode which changes
the power-on signal from DTR high to RX low (at the time we
started looking at modems for the HP 95, the Worldports were
the only ones with this feature). That is, the modem will
turn on when the RX line goes low. This seemed ideal for the
HP 95 since the RX line is held at ground normally (modem off)
and is driven low when an application which uses the serial
port is run (modem on), such as COMM.
This happens to work just fine for the Wordport 1200, but for
the 2400 and 2400MNP, when data is sent on the RX line, the
modem power tries to turn off and on with the data bits, rather
than remaining on. As you can imagine, this didn't work (this
mode was really not originally designed for this purpose). But,
not to worry, they were more than happy to make a minor change
in the design to fix that problem. Hence, the business about
"only modems after serial number such-and-such being able to
work on battery power" came into being.
Information we have received just the other day says that the
2400 has indeed been changed and works as advertized. The 1200
never really had the problem, so it always worked. However,
it appears that Touchbase/USRobotics never put the change into
production for the 2400MNP. So, THE WORLDPORT 2400MNP WILL NOT
WORK ON BATTERY POWER WITH THE HP 95, REGARDLESS OF THE SERIAL
NUMBER. Their customer support was apparently indicating to
some people that by putting the 2400MNP into "constant speed"
mode, it would work. This is not true. It appears to work
until you connect to the phone line. At that point, the added
current drain drags the power supply down and it either hangs
up or transmits garbage.
In addition to the power-on mode setting (dip switch number 3
ON), there is one additional setting which must be used to get
the modems to work with the HP 95. All modems monitor the DTR
line as an indication that there is a computer or terminal
there to receive the data. If DTR is disconnected or low, they
assume the terminal isn't there or has failed, and will usually
hang up and stop listening for commands. All modems have a
mode which tells them to ignore the DTR line, but it is generally
not the default, so the user must set this mode.
For the 1200, dip switch 4 must be ON to activate this mode (the
manual is totally confused, don't believe it). For the 2400 and
the 2400MNP, the user should send the modem command AT&D&W<cr>.
This only needs to be done once: it sets the DTR ignore mode and
writes that mode into the EEPROM so it will always come up in
that mode thereafter. THE DTR IGNORE MODE MUST BE SET IN ANY
MODEM IN ORDER TO WORK WITH THE HP 95, REGARDLESS WHETHER THE
MODEM IS ON BATTERY OR AC POWER.
So, to summarize:
1. In order to work with the HP 95, all modems must be set to
DTR IGNORE mode.
2. All the Worldport modems will work with the HP 95, provided
the modem is on ac power.
3. The Worldport 1200 (any serial number) and the Worldport 2400
(serial number 50000 and up) will work on battery power with the
HP 95 if the RX power on mode is set.
4. The Worldport 2400MNP will not work on battery power with the
HP 95.
Hope this helps....Steve