REVIEWS: How to Get Battery-Powered Portable Modems to Work with the HP 95LX

A portable modem expands the utility of the 95LX tremendously. Here's a little advice on using and choosing modems for the Palmtop PC.

By Larry Lefkowitz

WorldPort Modem in Cradle: Graphic

 Many HP 95LX owners are successfully using the palmtop as a portable communications platform to stay in touch with their office or on-line service while traveling. The 95LX with a battery powered pocket modem is an excellent, light-weight alternative to a notebook system.

However, there are several important points to consider if you want to use a portable battery-operated modem with the 95LX. What works on your PC or laptop may not work with your HP 95LX. This article highlights some of these differences, and presents a list of modems known to work with the HP 95LX. If you own a modem not mentioned on the list, the information presented here will be especially helpful.

Connecting Modem and 95LX

You must correctly connect your modem and 95LX before you can determine whether or not they will work together. Assuming that your pocket modem has a female 25-pin connector, a typical connection requires the following parts;

  1. 1. HP Serial Interface Cable -- HP 82222A ($35). Also available as part of the Connectivity Pack -- HP F1001A ($99.95).
  2. 2. 9-pin-to-25-pin Modem Adapter. Comes with the HP Serial Cable Adapter Kit -- HP 82224A ($15).



The 95LX connects to the serial cable, which connects to the modem adapter, which connects to the modem. If you don't have access to the HP modem adapter, you'll have to substitute three other adapters: The 9-pin-to-25-pin adapter that came with the Connectivity Pack, a 25-pin, male-to-male gender changer, and a 25-pin male-to-female null-modem adapter.

Again, you must properly connect your modem to your 95LX! None of the suggestions in this article will work unless a proper cable connection is first established.

The Unique HP 95LX Serial Port

Compare the serial port on your 95LX to one on a desktop or laptop PC and you'll notice that the 95LX's port is smaller and simpler. It only has four pins, three of which are used as Receive, Transmit and Ground wires. Using only three wires helps to conserve valuable battery power.

In contrast, your desktop PC's serial port has several additional wires (9-pin or 25-pin) that carry additional signals to your modem. Figure 1 below lists the pins and signals carried on them.

Figure 1

Pin # Name/purpose

2* TXD Transmit data signal (from computer to modem)

3* RXD Receive data signal (from modem to computer)

4 RTS Request to send signal (computer to modem)

5 CTS Clear to send (from modem to computer)

6 DSR Data Set Ready (from modem to computer)

7* GND Signal ground

8 CD Carrier Detect (from modem to computer)

20 DTR Data terminal ready (from computer to modem)

* These signals are provided by the HP 95LX. The remaining signals would normally be provided by a standard desktop/laptop system. Most of these additional wires are not critical to the operation of the modem. However, some portable modems require at least one of these additional signals to function properly. That is the Data Terminal Ready (DTR) signal.

DTR: The Missing Signal

The DTR (Data Terminal Ready) signal is one of the signals not provided by the 95LX that is particularly important for some portable battery-powered modems. Designers of portable modems were interested in conserving battery power. Many portable modems use the DTR signal coming from the computer to turn the modem battery power on and off. A modem waiting for the DTR signal from a 95LX will never receive it, and never turn on. Such a modem will generally not work with the 95LX, unless you can modify the hardware and fool the modem into thinking it is receiving the signal.

The hardware modification generally provides voltage to the DTR pin on your modem connector (pin 20). This voltage can be "borrowed" from another pin on the modem that has power during normal operation, or may be provided by an external power source (battery). The modems in the accompanying list which require modification are noted. One other thing to keep in mind; if you artificially supply the DTR signal in order to turn a modem on, you must remember to remove the signal so the modem can turn off. Otherwise, your battery will drain very quickly.

Software Command Sometimes Eliminates the Need for the DTR Signal

Some pocket modems will power on without the DTR signal, but WILL NOT function properly without it. Until the modem receives the DTR signal, it will remain in command mode and not pick up the phone line. Luckily, this problem can be overcome by sending the modem a simple software command. When this command is received, the modem will no longer require the presence of the DTR signal to function normally.

To send the command, follow these instructions: (It is recommended you issue this command to any pocket modem used with your HP 95LX.)

With your modem connected to the 95LX, go into the COMM program and press (MENU) Connect, then type AT &D0 &W and press (ENTER). If the modem receives the command properly, it will respond with OK. The "AT" tells the modem to get ready to accept a command. The "&D0" tells the modem to ignore the DTR signal. The "&W" records this command permanently in the modem's non-volatile memory (which means that you will not have to re-execute the command the next time you turn on your modem).

If your modem does not respond to this command with OK, you may need to use a PC or laptop computer to send the command. This is because some modems will not even accept a command to ignore the DTR signal without the DTR signal being present. Your PC or laptop can provide the DTR signal, while your 95LX cannot. Connect the modem to your PC, using a standard modem cable. Then, using a communications program on your PC, send the command as detailed above. The modem should now work properly with your HP 95LX.

Using Modems that Support Error Correction and Data Compression with the HP 95LX

Some modems support error correction protocols to insure the accuracy of data. If your modem supports MNP 2-4 or V42 error correction, and MNP 5 or V42bis data compression, you will need to enable "flow control" between your computer and the modem. Flow control allows the computer to regulate how much data is coming from the modem. Today's sophisticated compression technologies allows data to stream between the modem and computer at speeds up to 57K baud, which may overload the computers' ability to process it. Flow control allows the computer to signal the modem and, in essence, say: "Hey, wait a minute, I can't take any more data right now".

Conversely, flow control permits the modem to temporarily stop data coming from the computer. This may be necessary if the modem is encountering poor line conditions and needs to resend data more than once. While it is resending data, it cannot accept any more from the computer.

Not supported on the 95LX is the fastest, most reliable way to enable flow control. This method utilizes two special wires on the modem cable so that the Palmtop and modem can signal each other when they are, or are not, ready. RTS (Request To Send) and CTS (Clear to Send) signals are sent on pins 4 and 5 of the cable (see figure 1). A common term for this technique is "hardware handshaking".

A flow control method supported by the HP 95LX is accomplished through software. The computer and modem can send each other special characters (called XON/XOFF) in the normal stream of transmitted data. When using a modem that supports error correction/data compression with the HP 95LX, you must either enable XON/XOFF software flow control, or disable the error correction/data compression features of your modem.


To enable software flow control (XON/XOFF) on your HP 95LX, make a change in the configuration of your communications software. If you are using the built-in COMM program, follow these directions;

  1. 1) Press (COMM) on the 95LX to bring up the COMM program.
  2. 2) Press (MENU) Settings Config Other Flow XON-XOFF Enable



Save these configuration changes with the Save option on the Settings menu.

If you are using the COMMO shareware communications program, you must change two lines in your COMMO.SET file. You can use the built-in MEMO program, or other text editor to make the changes. When done, these two lines should look like this;



Many modems that support error correction/data compression have hardware flow control enabled as their default setting. A modem that is set for hardware flow control will dial normally when connected to the HP 95LX, but appear to freeze once connection is made with the remote computer. Consult your modem manual for instructions on how to change the modem's default setting to XON/XOFF software flow control. The command that works for many Hayes compatible modems is; AT &K4.

If you are using the built-in COMM program on your HP 95LX, you can enter the above command manually each time you turn on your modem. If you want to make this setting permanent in your modem, add &W to the end of the above command before executing it.

If you are using COMMO, just add &K4 to your modem initialization string in the COMMO.SET file.

Note: Certain file transfer protocols, such as Zmodem, require the use of hardware flow control, and will not work without it. Additionally, some communications programs require signals not provided by the HP 95LX (hardware handshaking, Carrier Detect) in order to operate properly.

Most AC Powered Modems Work

Most AC powered modems will work with the HP 95LX. However, their size, and the need to be close to an AC outlet, makes them inconvenient to use with the HP 95LX.

Most Line Powered Modems Don't Work With the HP 95LX

Line powered modems do not require a battery because they derive their power from both the phone line and the serial port of the computer they are connected to. While these modems work well with most desktop and laptop systems, the 95LX cannot provide them with the power they need to operate. There are a couple of line-powered modems, however, that can be made to work with the 95LX through hardware modification. See the modem list at the end of this article. If you are so inclined, you may find that you can modify your line-powered modem to work with the 95LX.

Get Information on 95LX Communications on CompuServe

The 95LX DataComm section (6) on CompuServe's HP Handhelds Forum (GO HPHAND) is an excellent place to share information on HP 95LX communications. If you have a question on getting your modem to work with the 95LX, there may be other users who can share their solutions with you.