Ideally, you should have your parallel port set for EPP mode. This is usually done via a BIOS setting in the system CMOS setup. But if your parallel port is on a card rather than part of the motherboard, then it may be done via a hardware jumper on the card. ECP mode (or any mode including ECP), while it sometimes works OK, is a potential source of trouble.
I've read that EPP mode may not work correctly if the parallel port is using address 3BC, so it would probably be best to have the port set to use either 378 or 278. Also, if you have more than one parallel port in the system, the installation may go more smoothly if the Zip is connected to LPT1. LPT1 is assigned to the port with the highest address, so setting the Zip's port to use 378 and the other port to use 278 might be best.
Be sure to connect your drive directly to the parallel port using the supplied cable, and make sure the cable is tightly connected at both ends. If you chain a printer to the passthrough port, then there's a possibility that its drivers could interfere with the Zip drive's operation. See this Iomega document for more info on that.
Chaining anything other than a printer to a Zip drive is generally not a good idea although some people have reported that they've been able to do so successfully. And be aware that the drivers for a device can interfere with a Zip drive's operation even when that device is not physically connected. So if you've ever had anything else using the same parallel port that the Zip drive is using, and if its software/driver has not been removed, this is something to look into if you're having problems.
If anything is chained to the Zip's passthrough port, do not try to access both devices simultaneously. For example, don't try to access the Zip drive while sending data to the printer, and don't try to print anything while transferring data to/from the Zip drive. Doing so could cause your computer to crash or perhaps even corrupt data on the Zip disk. Note that this may preclude printing files directly from the Zip disk; they must first be fully loaded into RAM or copied to the HD.
A parallel-port drive should not be turned on before the computer is turned on. Some systems won't even power up if anything connected to the parallel port has been turned on first. Iomega recommends using a power bar to turn everything on simultaneously. You could also power up the drive just after turning the computer on - before the OS begins loading. I use a power controller which sits under the monitor and gives me access to a power switch for just about everything in the system; that makes it easy to turn the Zip drive on right after turning the computer on.
Generally, Windows 9x works best when you're not loading any DOS drivers from Config.sys or Autoexec.bat. If you haven't read THIS, then you probably should. If you want those drivers for when you Restart the computer in MS-DOS mode, then there are better ways to load them. I think the best way is to edit the Exit To Dos PIF file in the Windows directory so that it loads everything you need for DOS mode from its Config.sys and Autoexec.bat boxes (as if from a floppy boot disk) which are enabled when you Specify a new MS-DOS configuration. See these Microsoft Knowledge Base articles for more info:
A parallel-port Zip drive uses the Iomega Parallel Port Interface (IPPI) which appears in Device Manager under SCSI controllers. The drive will be seen by Windows as a sort of virtual SCSI device.
If the Iomega software installation can't find your drive (or if you don't want to install the software), you can install the IPPI manually. Go to Add New Hardware in Control Panel. Select "No" when asked if you want Windows to search for hardware. Under Hardware types, select SCSI controllers (or perhaps Other devices). If you find the IPPI listed, then you can select and install it. If you don't find it listed, then use the Have Disk button to browse to an appropriate .inf file. It could be Ppa3.inf (on the Win98 CD-ROM) or Guest.inf (from an Iomega disk or download). You should then be able to select the IPPI from its list of drivers. Once the IPPI is installed, you should have basic read/write use of the drive (if it's connected and powered on). An alternative to using Add New Hardware would be to download the Ioware9xdrv.exe file from Iomega.
After the IPPI has been installed, try running Iomega's ppa3opt.exe (or ppaopt.exe) utility via the Parallel Port Accelerator shortcut on the Start menu to see what it comes up with for the Adapter settings. These can be found on the Settings tab under the Properties for the IPPI in Device Manager.
The Win9x miniport driver for the IPPI is Ppa3.mpd. You can find it using Explorer in the Windows\System\Iosubsys directory. Right-click on Ppa3.mpd; select Properties, and click on the Version tab to see what its File version is. Versions earlier than 2.22 are likely to provide much slower performance than the newer versions. If your installation has an older version, you can upgrade it to 2.34 by simply installing Ioware9xdrv.exe and rebooting for the change to take effect (see the Downloading Iomega Software section of this website). If you change your version of Ppa3.mpd, then you should run the Parallel Port Accelerator again to allow it to re-set the Adapter settings (some versions offer /mode: settings that previous versions didn't), and if those change, you'll again have to reboot for the changes to take effect.
If you're using Windows 98, then you may need to use Ppa3.mpd v2.23 (from the Win98 CD-ROM) or later. However, some people have reported having (printer-related) problems after installing v2.34 under Win98. If that happens, then copy v2.23 from the Win98 CD-ROM into the Windows\System\Iosubsys directory, and reboot for the change to take effect. Or it might also help to try v184.108.40.206 which comes with IomegaWare 2.x.
View the Properties/Settings for the Zip drive in the Disk drives section of Device Manager. Disconnect, Sync data transfer, and Removable should be checked there. No other boxes should be checked.
If you find that Int 13 unit is checked, then uncheck it, and reboot the computer. If you find that it has been checked again, then don't worry about it.
The Iomega document that addressed these "disappeared" when they reorganized their website (and it wasn't up-to-date anyway). So I thought I should include what information I can on them here.
The adapter settings should be in this form (but the exact settings will differ from system to system)...
/mode:nibble /port:278 /speed:6
You should try running Iomega's ppa3opt.exe (or ppaopt.exe) utility (there should be a Parallel Port Accelerator shortcut on the Start menu) to see what it comes up with for these settings on your system. If your Zip drive is not connected to LPT1, this may not work, and it doesn't always come up with the optimal settings anyway.
If you'll view Ppa3.mpd with a text editor, you should be able to make out the available mode settings near the end of the file. It looks like they are...
If you have an older version of Ppa3.mpd, some of these may not be available.
The smc* modes are for ports using the SMC chipset. The winb* modes are for ports using the Winbond chipset. The pc873* modes are for ports using the National chipset. sl360 is for ports with the Intel chipset. ecpecr and eppecr are apparently some kind of generic ecp & epp modes. And bidir and nibble also seem to be generic modes (nibble should be the slowest but most compatible setting). mcbidir is described as "Microchannel" (an old IBM bus type). And there are a few more modes you could try: EPP, ECP, and Fast. Supposedly, the Bidir, EPP, and Fast settings do not force the specified mode and can revert to nibble mode if necessary. Try EPP if you know you have an EPP port but Ppaopt.exe won't see it (or won't set it higher than bidir).
If you have a Zip Plus connected to the parallel port, you should use /plus: instead of /mode: for its mode setting. And if you're using a Jaz Traveller, I think it uses /jaz: for this.
The port setting would be the address of the LPT port to which the Zip is connected (you can look it up in Device Manager). EPP modes may not work correctly with ports using address 3BC.
The speed setting should be a number from 1 to 6 - with 6 being the fastest.
If you set any of these manually, you should probably run some tests after rebooting to make sure your drive is functioning correctly. Choosing the wrong settings for your system might result in anything from the drive not being recognized to data corruption; so be careful with these.
The DOS ASPI manager for the parallel-port Zip drive (and Jaz Traveller) will usually be ASPIPPM1.SYS. But depending on your hardware, it could be ASPIPPM2.SYS instead. Each of these has a number of .ILM files associated with it which are analogous to the mode settings in Win9x. There is also a SPEED setting. Choosing the correct .ILM file and SPEED setting can greatly improve performance in DOS mode. The OPTPPM1.EXE program attempts to find the optimal settings for your system, but they can also be set manually. See Manual.exe for more information.