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4610M-803-101103 Datasheet(PDF) 52 Page - Bourns Electronic Solutions |
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4610M-803-101103 Datasheet(HTML) 52 Page - Bourns Electronic Solutions |
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52 / 62 page  Specifications are subject to change without notice. 328 configuration (Fig. 1) for the single-ended implementation of the SCSI bus, or a three-resistor terminator configuration (Fig. 2) for the differential-line version of the SCSI bus. In the single-ended configuration, the SCSI bus is defined for lengths up to 6 meters, while the differential-line version pro- vides for better commonmode noise immunity over cable lengths up to 25 meters. The signal assignments on the single-ended SCSI bus include 8 data lines, 1 parity line, and 9 control lines. Each of these 18 lines must be terminated, and it is convenient to do this using a resistor network which contains all of the required resistors. An additional 32 lines are ground or power lines which do not require termination (there are 50 lines total in the cable). In a similar fashion, the differential configuration of the SCSI bus uses 18 pairs of lines, each requiring termination. As of 1989, an extended version of the SCSI standard has been in develop-ment by ANSI, called the SCSI-2 bus. This new standard allows for 16-bit to 32-bit wide data transfers, while also allowing a higher bit transfer rate. Two cables are defined in the SCSI-2 bus, termed Cable A and Cable B, where Cable B is optional (“wide SCSI” option). Cable A is no different than the single cable used in the original SCSI bus, and therefore uses the same number and types of resistive terminators (i.e., dual terminators for single-ended and triple ter- minators for differential). Cable B, however, is a 68-line cable, of which 29 lines (single- ended) or 29 line-pairs (differential) require termination. Application Guidelines The principles of transmission-line theory apply to SCSI termi- nators, and therefore for proper operation their placement must be restricted to the ends of the bus and nowhere else. This implies that the terminators should be placed as close to the SCSI devices as possible. It is permissible to place the termina- tor inside the SCSI device, but only if that device is located at the end of the bus. For disk drive applications, SCSI terminators must be present on the host adapter card and at the disk drive end as well. Many disk drive manufacturers have opted to design in removable SCSI terminators into their units to account for the possibility that their unit may not be the one at the end of the cable. For these manufacturers, the combination of a resistor network in a through-hole version plus a matching socket represents the only (and expensive) alternative. A final consideration is the cable itself. Since the terminators are comprised of 220 ohm and 330 ohm resistors (single-ended), the cable ideally should have a characteristic impedance which matches the Thevenin equivalent of this resistor combination, that is, 132 ohms. In the differential case, a characteristic imped- ance of 122 ohms would be ideal. In addition, it is inadvisable to mix different, unmatched cables in the same bus. Such a practice will result in undesirable signal reflections which may compromise the integrity of the data transfer. Bourns supplies a number of resistor network models designed for both SCSI and SCSI-2 termination . Cable A (SCSI and SCSI-2) Single-ended 330 Ω 220 Ω + 5V GROUND + SIGNAL FIGURE 1. 330 Ω 150 Ω GROUND - SIGNAL FIGURE 2. 330 Ω + 5V + SIGNAL 330 Ω 5V 220 Ω Dual Terminators 3V 132 Ω NO. PACKAGE REQ’D. BOURNS P/N DIP 1 4120R-3-221/331 CSIP* 2 4611X-104-221/331 CSIP* 3 4608X-104-221/331 MSIP* 3 4308R-104-221/331 Differential: Differential: NO. PACKAGE REQ’D. BOURNS P/N 4120R-820-1 DIP 2 4120R-820-2 DIP 3 4116R-8-2 CSIP* 3 4614M-8-2 MSIP 5 4310M-820-2 4120P-830-2 4420P-820-1 4420P-820-2 4420P-830-2 Cable B (SCSI-2 Only) Single-ended NO. PACKAGE REQ’D. BOURNS P/N DIP 2 4118R-3-221/331 CSIP* 3 4612X-104-221/331 CSIP* 4 4610X-104-221/331 MSIP* 4 4310R-104-221/331 NO. PACKAGE REQ’D. BOURNS P/N DIP 4 4118R-820-2 DIP 5 4116R-8-2 CSIP* 5 4614M-820-2 MSIP* 8 4310M-820-2 *MEDIUM PROFILE (.250” SEATED HEIGHT) AND HIGH PROFILE (.350” SEATED HEIGHT) ARE AVAILABLE BY PLACING THE LETTER “M” OR “H,” RESPECTIVELY, IN THE FIFTH POSITION OF THE PART NUMBER. SCSI Applications |
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