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Using PC Case to hold SCSI Devices for a Mac system

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Using PC Case to hold SCSI Devices for a Mac system.

The price difference between internal and external SCSI devices is about $50 to
$100. The cost to build a case to handle numerous internal SCSI devices can be
under $100. This will reduce the cost to add other devices. This report should
explain the cost, the material, and the work required to complete this task.

External SCSI devices come with a case, a small power supply, a loop-thru SCSI
connection, and maybe some type of termination. The PC case will provide the
mounting and the power supply for multiple SCSI devices. One ribbon cable
assembly is required to connect the devices together and to the rear of the
case. A shielded SCSI cable can be purchased to connect the Mac to the PC case.
The last device on the chain in the PC case will need to terminated. If other
SCSI devices that will not be in the PC case are used, they will need to be
located between the Mac and the PC case on the SCSI chain.

1)  PC Case - a mini-tower case with a 250W or more power supply can be
purchased for <$60 ($45 is a good price new). It should come with miscellaneous
hardware for mounting the devices. The power supply should be prewired with
connectors for providing power to numerous SCSI devices (typically 4 or more). 
2)  External SCSI Cable - a DB-25 to 50 pin centronics for about $15
3)  50 Pin Ribbon Cable - 5 feet or less. The quality is up to the buyer and
starts around $1/foot. (Digi-Key  R028-5-ND is $4.79 for 5 feet)
4)  50 Pin Centronics Connector - the external connector on the PC case at one
end of the ribbon cable. (CHAMP Latch Bail Lock - Amp # 553603-1; Digi-Key
A1569-ND $5.20)
5)  [Optional] Bail Lock Hardware - the hardware to lock the centronics
connectors together; other methods like screws can be used. (Amp # 552561-3;
Digi-Key A1517-ND $1.67)
6)  50 Pin Socket Conn with Polarizing Key - the connector that snaps onto the
ribbon cable where needed. If you have five devices, you need five connectors.
(Amp # 1-746286-0; Digi-Key AKC50T-ND $3.57)
7)  [Optional] Strain Relief for part #6 - at least one should be used for the
end of the cable, but the rest aren't needed if care is used. (Amp # 499252-4;
Digi-Key ASSR50-ND $0.77)
NOTE: Please verify these part numbers before ordering to make sure that the
proper part is ordered. Many different equivalent parts from other makers can
be used. Better quality (i.e. gold contacts) can be used to, but it increases
the cost.

COST:  (all cost are continental US dollars)
$ 60       PC Case 
$ 15       SCSI Cable 
$  4.79    Ribbon Cable 
$  5.20    Centronics Conn 
$  1.67    Bail Lock 
$ 17.85    Five Socket Conn 
$  0.77    One Strain Relief 
$105.28    Total (but your cost may vary)

SCSI CABLING: This is one area that usually worries most people out of the fear
of destroying their device. Buy an official Mac SCSI cable that is a 25 pin D
to 50 pin centronics. The rest of the way, just match up all of the pin 1's.
The other pin numbers will not match up due to the numbering convention. The
ribbon cable should be marked to show pin 1. Closely examine the 50 pin
connectors and look for the pin 1 marking (they all have them). Always keep pin
1's matched up and it will be correct. The connectors must be keyed to prevent
plugging them into the drive wrong. The ribbon connectors should only be
snapped on once so be careful. Reusing ribbon connectors usually results in a
less reliable connection and might cause opens or shorts. For putting the
connectors on the ribbon, try using a vise whose head is the length of the
connector. Uniform pressure along the connector helps make a good connection.
Also, make sure that the connectors are straight when putting them on the
ribbon to prevent misalignment. Before hand, figure out where you plan to mount
the drives and how the ribbon will be routed. Be sure and consider future
additions as you will use the same cable. All of the connectors don't have to
be put on the ribbon at the beginning. Leave some space and when the new drives
are added, put the connectors on then. This probably would require removing the
whole cable assembly to add the additional connectors. Be conservative in the
amount of ribbon cable used. Leave enough for additions, but too much can hurt
the SCSI bus. No more than 5 feet should be required. The maximum specified
limit length of the whole SCSI chain is 20 feet, but reality says that 15 feet
is a more practical upper limit. Use the SCSI reference table below if you
desire to double check the connections.

Centronics    Ribbon      DB-25    Signal     Signal   
  50 Pin      50 Pin      25 Pin    Name    Description 
----------    ------      ------   ------   ----------- 
  1,2,3       1,3,5        14       GND      Ground 
  4,5,6       7,9,11       16       GND      Ground 
 7,8,9,11   13,15,17,21    18       GND      Ground 
   10           19         nc       GND      Ground 
   12           23         nc       GND      Ground 
   13           25         nc        -       Open
   14           27         nc       GND      Ground 
   15           29         nc       GND      Ground 
16,18,19     31,35,37       7       GND      Ground 
   17           33         nc       GND      Ground 
20,21,22     39,41,43       9       GND      Ground 
23,24,25     45,47,49      24       GND      Ground 
   26            2          8       DB0      Data bit 0 
   27            4         21       DB1      Data bit 1 
   28            6         22       DB2      Data bit 2 
   29            8         10       DB3      Data bit 3 
   30           10         23       DB4      Data bit 4 
   31           12         11       DB5      Data bit 5 
   32           14         12       DB6      Data bit 6 
   33           16         13       DB7      Data bit 7 
   34           18         20       DBP      Data parity 
   35           20         nc       GND      Ground 
   36           22         nc       GND      Ground 
   37           24         nc       GND      Ground 
   38           26         25       TPWR     Term Power 
   39           28         nc       GND      Ground 
   40           30         nc       GND      Ground 
   41           32         17       ATN      Attention 
   42           34         nc       GND      Ground 
   43           36          6       BSY      Busy 
   44           38          5       ACK      Acknowledge 
   45           40          4       RST      Bus Reset 
   46           42          2       MSG      Message 
   47           44         19       SEL      Select 
   48           46         15       C/D      Common/Data 
   49           48          1       REQ      Request 
   50           50          3       I/O      Input/Output 
Note: The ground pinout may be different than the sample cable used to create
this table.

The power connectors on the power supply should be the standardized 4 pin keyed
connector and plug right into the SCSI devices. There will be some other power
connectors that can be ignored as long as they don't short out somewhere. Note
that some power supplies have open circuit cutback. A load on the 5 volt and
maybe the 12 volt line might be required to get the voltage outputs up to the
rated levels. This load may also be needed in the completed case if the
installed SCSI devices don't draw enough current. A 10 ohm >2.5 watt resistor
(P=(5^2)/10=2.5W) on the 5 volt line should provide enough loading. Most PC
cases have a few LEDs with two pin connectors to indicate power, turbo, and
reset. These can be connected to the 2 pin headers (usually found opposite the
50 pin connectors) on the harddrives to indicate disk access. The correct
polarity is required to get the LED to work, but the wrong polarity should not
damage anything. So just trying any polarity to see if works should be
acceptable. Most new PC cases won't even have the power switch pre-wired to the
power supply. The provided instructions should be used to complete this job.
Extreme care should be used when dealing with AC power lines.

The PC case should have plenty of mounting screws and brackets for supporting 4
to 5 SCSI devices. Be sure that the mounting doesn't distort the SCSI device's
case which could cause damage. Also make sure no loose cables or other objects
are allowed to cause a short on the device. Most harddrives will function just
fine no matter what the orientation. The key is to format the drives in exactly
in the same mounting used for regular drive operation. Remember that the last
SCSI device needs to be terminated. The mounting for the external centronics
connector is probably the hardest part of the whole deal. The connector doesn't
easily fit anywhere. A custom cutout in the back of the PC case is the cleanest
approach, but sticking it through one of the card slots is the easiest. Don't
let the ribbon cable stick through the case because the metal wall could wear
through the cables insulation cause a short.

The Mac SCSI allows 8 IDs. The ID 0 should be used by the internal drive and 7
should be the Mac. That leaves 6 others (1-6) that can be used for other SCSI
devices. The 6 pin (2 rows of 3 pins) header on the devices is used to set the
ID and is a three bit binary number. Only one device can be used for each ID or
the bus will not function and might cause loss data. Note that if more than one
device arbitrates for use of the bus at the same time, the device with the
higher ID gains control first. Once a device, regardless of ID, gains control
of the bus, no other device can interrupt that session.

The first and last device (by physical cable location, not by ID) in the SCSI
chain must be terminated. The internal harddrive should be internally
terminated. A row of 24 pin sockets is typically located on most harddrives
near the 50 pin connector. These sockets hold the line termination resistor
arrays used for SCSI termination. If the resistors are in, the device is
considered internally terminated. If not, then it is unterminated. Some reports
state that some exceptions exist to the termination rule. If the SCSI chain is
<18 inches, then terminate one end. If the chain is >10 feet, then a terminator
in the middle should be added. Always terminate just both ends first and only
change if problems exist. Three 8 pin 220/330 ohm line termination resistor
arrays are typically used to provide the termination. The faster the mac is and
the more devices there are attached to it, the more important termination is.
[Some info from Mac Bible]

While the SCSI bus should be a plug-n-play interface, some problems have been
noticed in different configurations. Some devices will not function properly
with other devices. The physical order on the SCSI chain sometimes causes
problems with some devices. This SCSI PC case should change any of these
conditions. If the devices functions properly in a standard setup using
individual cases and cables, then it should work in the PC SCSI case.

The shielding of the case and the ribbon cable is a concern that many people
have. External devices have metal or conductive walls and have real short
ribbon cables. The PC case should provide the necessary shielding required for
the outside world. The major question is the shielding requirement from device
to device and from cable to device. For most systems (almost all home systems),
this PC case system will provide plenty of shielding and the SCSI communication
will function properly. The PC case system may cause SCSI re-tries and other
problems with the SCSI bus if not properly implemented. The use of a SCSI test
program can be used to conduct timing tests to see if the PC case system is not
performing properly.

1.  Heat Dissipation - If you think (or notice) the case will get too hot then
add a fan. The speaker mounting might be a good place to mount a fan. Consider
the air flow as some devices should not suck in outside air due to dust. A DC
fan could easily be powered by the internal power supply. The PC power supply
should be able to handle the 4 or 5 devices without a problem. Check the power
consumption of each device to be sure.
2.  SCSI Loop Thru - You can make this system with a loop thru by adding an
external connector at the end of the ribbon cable. This greatly increases the
length of the cable and slightly increases the cost and work, but it is
definitely doable. This may be required if you have another device that must be
at the end of the SCSI chain.
3.  On/Off Power - All of the devices mounted in the PC case will get powered
on when you power up the supply which is a feature of this system. Extra 2P2T
switches could be wired up to the power of certain drives to provide any
additional on/off control that is desired. Note that some devices must be on
for the SCSI bus to work at all. The SCSI case should also be powered on prior
to the computer and allow to come up to speed.
4.  Device Verification - Each SCSI device should be verified in the system
without any other devices connected. Make sure that each one works and then add
them one by one until your system is complete. A lot of wasted debug time can
be spent on a system when just one device is defective.
5.  Risk/Guarantee - If you don't have the money to replace anything that you
damage, then don't attempt to make this SCSI case. While the system is
relatively simple to complete, any time you handle computer devices there is a
risk factor. Some (or many) stores or dealers may not stand by their warrantee
if you use their device in this non-standard system. It is also difficult to
get salesmen to even understand the setup.
6.  Apple CD300i - This drive will easily mount in one of the 5" slots of the
PC case. No extra mounting hardware is required even a salesmen will try to
sell you one. The device will fit flushly on the front and doesn't need a cover
plate. A unique cable will need to be made to get the rear audio output mounted
on the back of the PC case. The front mini-headphones connector should be good
enough for most users. Note that some internal CD drives will not have the
front connector or the volume control.
7.  SCSI Speeds - The faster the SCSI speeds, the better the SCSI cabling must
be to prevent problems or delays. This PC case system is not the best, but it
is probably better than some SCSI chains out there already. It should not cause
any noticeable difference in your SCSI performance. The quality of each system
depends on how it is put together and what devices it uses. This PC case should
be acceptable for most everyone. If you are really worried about your SCSI
speed, then don't attempt to build and use this system.
8.  Other Jumpers - There will probably be some other jumpers on the
harddrives. Don't change their configuration unless you exactly what you are
doing. Drive manufacturers should be able to provide any additional information
on the extra jumpers.

The SCSI PC case is a great way to add devices and minimize the cost. This
report should be a valuable aid to anyone considering the idea. You should
think everything through clearly for yourself before attempting this adventure.
No promises are made. Other people have completed the same type of system with
success. Good luck.