DS2108 differential scsi switchable terminator DS2108 preliminary 022698 1/5 features ? fully compliant with scsi, scsi2 and scsi3 stan- dards ? conforms to eia rs485 standard ? provides differential termination for 9 pairs of signal lines ? operates with scsi signal voltages of 7 to +12 volts ? lasertrimmed 330 and 150 ohm termination resis- tors have 5% tolerance over full temperature range ? switchable power down mode ? low power down capacitance of 6 pf ? 24pin plastic soic (DS2108s) pin assignment 1 2 3 4 5 6 7 8 9 10 11 12 24 23 22 21 20 19 18 17 16 15 14 13 pd gnd r1p r2p r3p r4p r5p r6p r7p r8p r9p gnd tpwr r1n r2n r3n r4n r5n r6n r7n r8n r9n tpwr nc DS2108s 24pin soic (300 mil) description the DS2108 scsi terminator has been specifically designed for high voltage differential (hvd) scsi sys- tems requiring switchable termination. the DS2108 integrates eighteen 330 ohm and nine 150 ohm precise switched termination resistors into a monolithic ic. the surface mount soic package saves board space over conventional resistor sips. the termination resistors can be isolated from the scsi bus under software or hardware control. while in the powered down mode, the DS2108 isolates the 9 terminator blocks from the bus while adding only 6 pf capacitance to each signal line of the scsi bus. functional description the DS2108 consists of power down circuitry, eighteen 330 ohm and nine 150 ohm termination resistors (figure 1). the DS2108 can be removed from the circuit by bringing the power down pin (pd ) low. the power down capacitance on the terminating resistors is 6 pf, well below the scsi3 allotment of 25 pf. the DS2108 sup- ports scsi signal voltages of 7 to +12 volts when pow- ered on or off. when all lines settle into the quiescent state (no signal transitions), 56 ma is typically con- sumed. only 1 ma is typically consumed in the powered down mode.
DS2108 022698 2/5 DS2108 block diagram figure 1 pd tpwr gnd r1p r9p power down circuitry, charge pump 330 ohm 150 ohm 330 ohm 330 ohm 150 ohm 330 ohm r1n r9n 50k ohm
DS2108 022698 3/5 typical differential scsi bus configuration figure 2 pd 2.2 m f 0.01 m f DS2108 DS2108 DS2108 driver receiver gnd tpwr r1p r1n pd gnd tpwr r1p r1n pd gnd tpwr r1p r1n driver receiver 2.2 m f 0.01 m f termination power line notes: 1. two DS2108s required per 8bit scsi device and three DS2108s per 16bit scsi device. 2. midbus termination effectively removed by grounding pd pin. 3. termination power to be provided as specified in scsi3 parallel interface (spi) document. 4. local termpwr bypassing is recommended by the spi document with values shown. the bypass capacitors should be located as close as possible to the DS2108s. only one pair of capacitors is required per scsi device (may be shared between DS2108s on same device). pin description table 1 pin symbol description 1, 11 tpwr termination power. should be connected to the scsi termpwr line. bypass with a 2.2 m f cap in parallel with a 0.1 m f cap as shown in figure 2. 2, 3, 4, 5, 6, 7, 8, 9, 10 r1n. . .r9n signal termination negative. connect to signal of scsi bus. 12 nc no connect. do not connect any signal to this pin. 13, 23 gnd ground. signal ground; 0.0v. 14, 15, 16, 17, 18, 19, 20, 21, 22 r1p...r9p signal termination positive. connect to +signal of scsi bus. 24 pd power down. when tied low, the DS2108 enters a powerdown mode. contains an internal 50k pullup. strap low to deactivate the DS2108, leave open circuited to acti- vate the DS2108.
DS2108 022698 4/5 absolute maximum ratings* voltage on any pin relative to ground 7.0v to +12.0v operating temperature 0 c to 70 c storage temperature 55 c to +125 c soldering temperature 260 c for 10 seconds * this is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation sections of this specification is not implied. exposure to absolute maximum rating conditions for extended periods of time may affect reliability. recommended operating conditions (0 c to 70 c) parameter symbol min typ max units notes termpwr voltage v tp 4.00 5.25 v pd active v pda 0.3 0.8 v pd inactive v pdi 2.00 v tp +0.3 v dc characteristics (0 c to 70 c) parameter symbol min typ max units notes termpwr current i tp 56 65 ma 1, 6 powerdown current i pd 1 2 ma 1, 5 command mode voltage v em 7.0 12.0 v 7, 8 termination resistance r ttn 313.5 330 346.5 ohms 2 termination resistance r tnp 142.5 150 157.5 ohms 3 termination resistance r tpg 313.5 330 346.5 ohms 4 powerdown termination capacitance c pd 5 6 pf 1, 5, 7 notes: 1. 4.00v < termpwr < 5.25v 2. 330 w resistor between termpwr and signal. 3. 150 w resistor between signal and +signal. 4. 330 w resistor between +signal and gdn. 5. pd = 0.0v. 6. signal pins left open. 7. guaranteed by design; not production tested. 8. signal lines rxn and rxp only.
DS2108 022698 5/5 24pin soic (300 mil) the chamfer on the body is optional. if it is not present, a terminal 1 identifier must be positioned so that 1/2 or more of its area is contained in the hatched zone. pkg 24pin dim min max a in. mm 0.094 2.38 0.105 2.68 a1 in. mm 0.004 0.102 0.012 0.30 a2 in. mm 0.089 2.26 0.095 2.41 b in. mm 0.013 0.33 0.020 0.51 c in mm 0.009 0.229 0.013 0.33 d in. mm 0.598 15.19 0.612 15.54 e in. mm 0.050 bsc 1.27 bsc e1 in. mm 0.290 7.37 0.300 7.62 h in mm 0.398 10.11 0.416 10.57 l in mm 0.016 0.40 0.040 1.02 q 0 8
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