Using Jaz Traveller with non-Iomega drives

Overview

This page describes how to use the Iomega Jaz Traveller with arbitrary SCSI devices (including the Iomega Jaz removable disk drive).
Jaz Traveller is an external SCSI hostadapter connected via IEEE-1284 port (aka "parallel" printer port) to the host. It can control a standard SCSI bus with up to 8 devices like any other SCSI hostadapter. Restrictions to Iomega drives or limited device count are done by Iomega software only.

Jaz Traveller SCSI hostadapter

In the picture above an adapter is used to connect the Jaz Traveller to the "Centronics" style SCSI port of the external SCSI case. A SCSI terminator is connected to the second connector (the other end of the SCSI bus).

Technical details

To say it first: It was not Iomegas day when they designed the cabling for the Jaz Traveller. Look at this picture:

Jaz Traveller cabling

On the right you can see a passive adapter from the Laptops IEEE-1284 printer port to a standard 50Pin SCSI cable with "HD" connectors. On the left you can see the Jaz Traveller SCSI hostadapter connected between the other end of the cable and the first device on the SCSI bus (only one is present in this configuration). What I don't like is that Iomega use the 50Pin standard SCSI cable to transfer IEEE-1284 signals ... if you simply forgot the Jaz Traveller the cable will also fit but you have made a totally incompatible connection between IEEE-1284 and SCSI that can possible destroy the port on the host. It would be better if Iomega had placed the Jaz Traveller before the SCSI cable using one 25Pin and one 50Pin connector (the passive adapter would be not needed in this case so it would be cheaper too).

If you look inside the Jaz Traveller you see a single ASIC that do the whole work:

Jaz Traveller PCB

A SCSI terminator is integrated in the ASIC, this means that the Jaz Traveller must be on the end of the SCSI bus and no additional terminator is required for this end of the SCSI bus. The other end of the SCSI bus must be terminated as usual.
As you can see there is no connector for an external power supply and the IEEE-1284 port of the host do not supply power. This means that the SCSI bus connected to the Jaz Traveller must provide a power source for the TERMPOWER line. This power source will supply the terminators as well as the Jaz Travellers logic circuit.

The whole construction looks like this:

------+
      |       +-----------+                            +-----------+
      |       |           |    IEEE-1284 signals on    |           |         SCSI bus
      |       |  Passive  |    standard SCSI cable     |    Jaz    |          +---+          +---+
 Host |-------|           |----------------------------|          T|----------| D |--- ...---| T |
      | 25Pin |  Adapter  | 50Pin                50Pin | Traveller | 50Pin    +---+     ^    +---+
      |       |           |                            |           |                    |
      |       +-----------+                            +-----------+       Up to 6 additonal SCSI devices
------+

D: SCSI device, T: SCSI terminator

Power source for TERMPOWER

I have used the Jaz Traveller to connect a Traxdata CDR4120 CDROM drive to a Toshiba T2130CT Laptop:

Many SCSI drives can provide TERMPOWER by simply setting a jumper. Unfortunately the CDR4120 cannot provide TERMPOWER so I have to switch on the soldering iron ... I have added a 2A fuse in series with a SB530 (5A Schottky diode) from the 5V output of the power supply to the TERMPOWER line of the SCSI bus:

TERMPOWER patch for external SCSI case

Note: The connector in the picture above is for the internal backplane of the SCSI case and therefore does not have the standard SCSI pinout! Look at the schematic below to find the TERMPOWER pin on the SCSI connector.

Schematic:

                                             5A Schottky diode

                               2A Fuse                +-
        Power supply        +-----------+          |\ |              SCSI bus
5V >------------------------|-----------|----------|->|--------------------------> TERMPOWER
                            +-----------+          |/ |
                                                     -+

                                                   SB530

TERMPOWER pins for common "narrow" (8Bit) SCSI cables and connectors:
50Pin ribbon cable        : Pin 26
50Pin IDC connector       : Pin 26
50Pin Centronics connector: Pin 38
50Pin HD connector        : Pin 38

It is obvious that the diode must be able to conduct more current then the fuse. Do not use a normal silicon diode because of the higher voltage drop compared to Schottky diodes. The value of 2A for the fuse was chosen as follows:
A "narrow" SCSI bus has 18 signal lines that must be terminated. This means that there are 36 terminators (18 on each side of the bus) present that must be supplied by the TERMPOWER line. Each terminator draws a current of 26mA from TERMPOWER at worst case. For all terminators together this leads to a maximum current of 936mA. The supply current of the Jaz Traveller will add to this value but should not be significant against the terminators. 2A is therefore a limit twice the normal worst case conditions and the fuse will not immediately be blown if you connect 3 terminators to the bus by accident.

Drivers

As I have written above, the Iomega driver disk that is shipped with the Jaz Traveller is useless if you have non-Iomega drives connected to the Jaz travellers SCSI bus. If you start the "GUEST.EXE" program you only get something like "No Iomega drives found".
Fortunately normal hostadapter drivers for the Jaz Traveller exists. I have used the one that is shipped with Windows 98.
There is also a Linux driver called "ppa" that should support the Jaz Traveller.