Adding an External SCSI Connector to the A4000.

Introduction

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The A4000 does not have a SCSI controller fitted as standard although there are a wide choice of third-party boards available. These come in two types: controllers which fit into a Zorro slot, for example the Octagon, and those that are integral to an accelerator card, for example the WarpEngine. Zorro controllers usually have a socket fitted enabling SCSI external peripherals to be plugged straight in. Controllers like the WarpEngine which replace Commodore's CPU card often only have an internal 50 way socket.

I wanted to be able to plug Iomega's external SCSI ZIP and Jaz drives into my WarpEngine SCSI controller..

There are several alternative "standard" SCSI connectors but I decided to fit a DB25 socket. These are fitted to Apple Macs and to many Amiga cards including the GVP range. Warren Block in his excellent A4000 Hardware Guide suggests the use of the Centronics in preference to the Mac connector but I have never experienced problems using the DB25 way socket with short cable runs.

To ensure correct operation the first and last devices on the SCSI chain should be terminated with the termination off on any intermediate devices. If you fit an external SCSI connector as described here this means the controller and the externally connected device should be terminated while the internal drives are unterminated. If you intend to remove your SCSI external device it should be replaced with a seperate terminator, available from most Mac suppliers.

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In practice I found my setup was very uncritical of the actual terminations and I was able to leave the termination on my hard drive on. The controller does not care if the external device is connected or not, or terminated or not. It just works! For further information on SCSI termination see Warren Block's A4000 Hardware Guide

I am pleased to answer any questions you may have, email to pwhatton@enterprise.net

I know of three ways to add an external SCSI connector.

• Centronics 50 way crimp SCSI connector.
• Hack an old A590. It has all the parts you need!
• Solder a DB25 on the cable

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Fitting a DB25 direct onto the cable.

This method is cheap (free is you already have a DB25) and dirty. But it works.

Parts Needed

• A length of 50 way IDC (SCSI) ribbon cable (up to 1 metre)
• Sufficient 50 way IDC plugs for your controller and each internal SCSI device
• A 25 way "D" socket

Method

Do not be tempted to re-use old IDC connectors, they are almost inpossible to remove from a ribbon cable without breaking the plastic and anyway they are designed to be used only once. Re-using them can lead to unreliable connections. It is much better to buy new parts and leave your old SCSI ribbon cable intact. If the new cable doesn't work you can always re-fit the original one.

Ensure that the power is off and the mains lead is removed from the A4000 before starting. Remove the case and undo the four screws which fix the hard drives into the rear bay.

Remove the small plate which covers the "Expansion" port on the A4000 rear panel. You can either make a cutout for the DB25 way socket using this plate or preferably find a back plane from an old PC or Amiga card which has the correct cutout already made and trim it to the correct length.

Start making the cable by crimping one of the 50 way IDC connectors onto the end of the new ribbon cable. This is the end that will connect to the socket on the accelerator card. Ensure that the (red) identifying stripe is crimped next to pin 1. There are several methods for crimping on the connector if the correct tool is not available. The connector can be squeezed onto the cable between the jaws of a vice using wooden blocks to prevent damage or, with care, a large pair of pliers can be used.

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Form the cable to shape so that it reaches each of the devices on you internal SCSI chain. In my case the cable was routed from the accelerator/controller to a CD ROM drive and then to the SCSI hard-drive in the rear bay. Between the CD ROM and the hard drive I had to put two 90 degree folds into the cable to line up the connectors the correct way (pin 1 is almost always next to the power connector on a CD ROM or hard drive)

Crimp IDC connectors onto the cable in the correct position for each device. Take care to make sure the connector is facing the correct way in each case.

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The cable should be run underneath the hard drive and out to the Expansion Connector on the back panel.

Cut off the ribbon cable so it is a few inches longer than needed and seperate each conductor, peeling it back about 1 inch (2.5cm).

Counting from pin 1,the red stripe on the ribbon, all odd numbered wires are grounds and can be folded back for the moment.

The data lines etc should be soldered neatly onto the DB25 socket as follows, information courtesy Warren Block's A4000 Hardware Guide

SCSI Pin-Outs

  Internal SCSI Port (50-Pin Header)

All odd numbered pins are grounds, except for pin 25, which is unused.

   Pin 2: *Data 0
   Pin 4: *Data 1
   Pin 6: *Data 2
   Pin 8: *Data 3
  Pin 10: *Data 4
  Pin 12: *Data 5
  Pin 14: *Data 6
  Pin 16: *Data 7
  Pin 18: *Parity
  Pin 20: Ground
  Pin 22: Ground
  Pin 24: Ground
  Pin 26: Terminator Power
  Pin 28: Ground
  Pin 30: Ground
  Pin 32: *ATN
  Pin 34: Ground (Unused?)
  Pin 36: *BSY
  Pin 38: *ACK
  Pin 40: *RST
  Pin 42: *MSG
  Pin 44: *SEL
  Pin 46: *C/D
  Pin 48: *REQ
  Pin 50: *I/O
 

External SCSI Connector (DB25 Macintosh-Type Female Pseudo-SCSI)
(Note: This is by far the most common style of DB25 SCSI connector. Most [probably all] Amiga SCSI interfaces with DB25 connectors--like the A2091--use this pin-out.)

   Pin 1: *REQ
   Pin 2: *MSG
   Pin 3: *I/O
   Pin 4: *RST
   Pin 5: *ACK
   Pin 6: *BSY
   Pin 7: Ground
   Pin 8: *Data 0
   Pin 9: Ground
  Pin 10: *Data 3
  Pin 11: *Data 5
  Pin 12: *Data 6
  Pin 13: *Data 7
  Pin 14: Ground
  Pin 15: *C/D
  Pin 16: Ground
  Pin 17: *ATN
  Pin 18: Ground
  Pin 19: *SEL
  Pin 20: *Parity
  Pin 21: *Data 1
  Pin 22: *Data 2
  Pin 23: *Data 4
  Pin 24: Ground
  Pin 25: Terminator Power
 

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The extra ground wires from the ribbon cable should all be connected to a suitable point. They can be soldered directly to the shell of the DB25 socket. Double check all your connections, it is very easy to wire the DB25 incorrectly Once you have made up the cable you can carefully reassemble the computer and test the SCSI external connector by plugging in your peripheral device.

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