Click to enlarge image.
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 in Iomega's external SCSI ZIP and Jaz drives and after hunting around unsucessfully for a suitable adaptor cable decided to build one myself.
No drilling or cutting of the A4000 case is required for this hack which is easily reversible. Soldering skills are required.
This hack has been built and tested by the author and is supplied in good faith. The author cannot be held responsible for loss and/or damage resulting from the use of the information contained here. If in doubt do not attempt this modification.
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There are several alternative "standard" SCSI connectors including the
Centronics 50 way but I decided to fit a Mac type DB25 socket. These are
fitted to most 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 any problems using the DB25 way socket with short cable runs.
Parts Needed
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
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?)
TT>
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
TT>
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
Click to enlarge image
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.
Termination
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.
Details of how to build an external SCSI terminator are available from the Aminet:
SCSITerminator.lha hard/hack 53K External SCSI Terminator
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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 seem to care if the external device is connected or not, or terminated or not. In my case it just works whether the external drive is plugged in or not. 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