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. INTRO: 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. CONCEPT: 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. MATERIAL: 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. SCSI CABLE PINOUT: 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. OTHER CABLING: 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. MOUNTING: 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. SCSI IDS: 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. TERMINATION: 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] SCSI DEVICE ORDER: 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. SHIELDING: 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. POSSIBLE QUESTIONS/PROBLEMS/WORRIES: 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. COMMENTS: 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.