Interfacing the TAPR 9600 bps Modem with the Ottawa PI2 Packet Interface Card

by Steven R. Bible, N7HPR
May 28, 1995 (revised July 15, 1995)

Reprinted from the TAPR Packet Status Register, Issue #59, pp. 6 - 8, Summer 1995.


This article describes the interfacing of a TAPR 9600 bps modem with the Ottawa PI2 Packet Interface card. There are several choices you can make in interfacing the two. Throughout this article I will introduce those choices and focus on the optional RS-422 interface of the Ottawa PI2 card with the TAPR 9600 bps modem.

The TAPR 9600 bps modem was designed to be interfaced with a host TNC via the TNC's modem disconnect header. The host TNC then supplies the necessary signals, power, and housing for the TAPR modem. However, there is no provision for installing a high speed modem on the Ottawa PI2 card, aside from the optional 1200 bps AFSK modem. Therefore, the TAPR 9600 bps modem must be installed external to the PI2 card. This will require a wire interface and the TAPR modem will require its own power and enclosure.

Interfacing the PI2 card with an external modem is done via one of two ports. The A port is the high speed port capable of interfacing with high speed modems such as the GRAPES WA4DSY 56 kbps, Kantronics D4-10 (19.2 or 9.6 kbps), or the TAPR 9600 bps modem. Connection with the external modem can be done using TTL or RS-422 signal levels. The B port is a low speed port capable of upto 9600 bps using a reasonably fast computer. Signal levels coming from the B port are TTL only. The B port can also be interfaced directly to a radio when the optional 1200 bps AFSK modem is installed on-board the PI2 card. Consult The PI2 Packet Interface User's Guide for more information.

In this article I will describe the interfacing of the A port with the TAPR 9600 bps modem. Signal levels coming from the A port are by default TTL, and the maximum recommended distance between TTL devices is 18 inches. However, many people have been successful with longer distances. You can experiment and see if interfacing an external modem via TTL levels will work for your particular situation. Choosing TTL signal levels at a considerably long distance could be fraught with erratic performance, especially if there is excessive Radio Frequency Interference (RFI) from computers and transmitters in the area.

Interfacing the PI2 card with an external modem via the TTL signal levels is the easiest option, though not the most optimal. The short distance of TTL limits the choices of where to place the modem in a typical installation and TTL is prone to interference. The solution is to install the optional RS-422 interface onto the Ottawa PI2 card. This will extend the distance an external modem can be placed and provide for a high speed interface that is more resistant to interference than TTL. However, the TAPR 9600 bps modem does not have a RS-422 interface option. It was designed for TTL levels from the modem disconnect header of the host TNC. Therefore, a daughter board needs to be constructed for the TAPR modem to interface the TTL signals with RS-422 signals coming from the Ottawa PI2 card.

Installing the Optional RS-422 Interface to the Ottawa PI2 card

Appendix D in the PI2 Packet Interface User's Guide that is provided with the PI2 card explains how to install the RS-422 option on the card. To summarize from the manual, to install the RS-422 interface, do the following:

Housing the TAPR 9600 bps modem

Construct the TAPR 9600 bps modem according to the instructions provided with the kit. Follow the directions for "TNC 2 Internal Installation" on page 8 of the manual. It is recommended that the clock option be installed on the modem. It is an inexpensive option and it simplifies interfacing the PI2 card with the TAPR modem.

Note: Throughout the discussion below, always refer to the TAPR 9600 bps modem manual for guidance. Make sure you understand why the changes listed below are being made. Blindly following directions without understanding why can lead to disaster! Double check all connections against the schematics supplied with the TAPR modem and the PI2 card.
The TAPR 9600 bps modem was originally designed to plug into the modem disconnect header of a host TNC. The TNC supplies the necessary TTL signals, power, and a housing for the modem. Since the TAPR modem is not being installed inside a TNC, these items need to be supplied.

Housing - An aluminum project box 8" x 4" x 2" (LMB No. 842) is an ideal size. There is plenty of room for the modem with a 1/2 inch clearance around it. This will allow for three LEDs (DCD, PTT, and PWR), power switch, fuse, 15-pin female D connector, 5-pin female DIN connector, and a power connector to be mounted on the box. Install the modem in the box using three 3/4 inch stand-offs. This will allow sufficient clearance for the RS-422 interface daughter board explained later.

Power - Install U23, the LM7805 voltage regulator on the TAPR modem. The modem draws approximately 50 mA. A power cube of the appropriate current rating at 12 vdc will supply the necessary power to the modem. It is recommended that a power on/off switch and a 1/2 Amp fast blow fuse be installed on the project box for safety. Apply positive 12 vdc power to the modem via header pins 24 and 26 of P3. You may find that the LM7805 will require a heat sink when powering the RS-422 interface chips on the daughter board explained later.

Signals - The TAPR modem expects TTL level signals at the modem disconnect header (P3). To convert these signals into RS-422 requires the construction of a daughter board that will plug into P3 of the TAPR modem. P3 is a 20-pin female header that is soldered to the underside of the modem.

Constructing a RS-422 daughter board for the TAPR 9600 bps Modem

The daughter board will contain a RS-422 driver chip (26LS31), receiver chip (26LS33 or 26LS32), 20-pin male header, two resistors, and two bypass capacitors (0.1 uF). The PC board is a Radio Shack mini PC project board (276-148). Solder the components to the project board according to the placement diagram and schematic below. When finished, the 20-pin male header of the daughter board will plug into the bottom of the TAPR modem's 20-pin female header. Connect the daughter board to the 15-pin female D connector on the project box using a ribbon cable. This will lead to the PI2 card.

Power for the two chips on the daughter board can be taken from the TAPR modem. There are three blank chip positions on the TAPR modem that are reserved for the bit regenerative option. Tap into pin 16 of U3 for 5 vdc.

As mentioned earlier, you may find that the RS-422 interface chips will add an extra load on the TAPR modem's LM7805 voltage regulator. This may require adding a heat sink to the LM7805.

Interfacing the TAPR 9600 bps modem with the Ottawa PI2 card

Now construct an interface cable. A 36 inch long (approximately 1 meter) cable will do fine for most installations. The cable requires at least 9 conductors and it is preferred that it be shielded to help reduce interference.

It was chosen to install a 15 pin female D connector on the aluminum box that houses the TAPR modem. This was to match the 15 pin female D connector on the PI2 card. This way two 15 pin male D connectors can be installed on both ends of the interface cable. Provided the connectors are wired pin-for-pin, you do not have to worry about which connector goes where. Below is the wiring diagram for the interface cable.

Interface Cable Wiring Diagram

PI2 card      Description      TAPR modem
--------      -----------      ----------
  P1-6           TXD(+)          P1-6
  P1-14          TXD(-)          P1-14

  P1-7           RTS(+)          P1-7  (*)
  P1-15          RTS(-)          P1-15 (*)

  P1-3           RXD(+)          P1-3
  P1-11          RXD(-)          P1-11

  P1-5           DCD(+)          P1-5
  P1-13          DCD(-)          P1-13

  P1-9           Ground          P1-9

(*) Note: See text for optional open collector configuration.

Interfacing the TAPR Modem to the Radio

A 5-pin DIN female jack was installed on the project box for the TAPR modem to Radio interface. This is the standard TNC-2 type radio port connection. This allows connectors already made up for other radio/tnc pairs to be used in this project. Below is the pin layout for the radio port.

Radio Port Wiring Diagram

TAPR Modem P1   Description   5-Pin DIN Jack
-------------   -----------   --------------
     1              RXA             4
     2              TXA             1
     3              GND             2
     4              GND             2
     5              PTT             3

Optional RTS Open Collector Interface

The Request To Send (RTS) signal commands the TAPR modem to transmit. The interface described above uses a RS-422 signal level for RTS. There is one draw back to this design. The modem needs to be powered off before the computer, otherwise the modem will go into constant transmit until the modem's watch dog timer times out.

To remedy this situation you can change RTS to an open collector signal level. The RTSAA jumper (J4) on the PI2 card allows for RS-422 or an open collector signal for RTS. To change the RTSAA signal to open collector, move the jumper on J4 to the upper two pins. This changes the RTS signal coming from the PI2 card (P1-7) to open collector signal level. At the TAPR modem, the RTS signal should be passed directly to the modem disconnect header P3 pin 5. Now the problem of constantly transmitting when the computer is turned off will be solved.


Together, the TAPR 9600 bps modem and the Ottawa PI2 Packet Interface card make a great price/performance combination. The PI2 card provides the high speed DMA driven port interface, and the TAPR modem the low cost high speed modem. The TAPR modem was designed to be interfaced with a host TNC via a modem disconnect header. The high speed port A of the PI2 card can only be interfaced to an external modem. This article describes how the two can be interfaced to provide the hardware component of a packet radio system. This hardware coupled with KA9Q NOS or the Linux Operation System completes the system and provides the user with a highly capable network node in the amateur packet radio network.