Note 073: DS2107A Active Negation This application note describes
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Application Note 399: 2001
Note 073: DS2107A Active Negation
This application note describes active negation need SCSI terminators, such DS2107A, able handle loads active negation imposes. DS2107A used SCSI systems provide active termination signal lines. typical bitwide data configuration cable), DS2107A's required fully terminate control lines data lines parity line). 16-bit wide data configuration cable), three DS2107A's required fully terminate control lines data lines parity lines). packages available DS2107AS, 16-pin SOIC, DS2107AE, 20-pin TSSOP (Thin Shrink Small Outline Package).
What Active Negation?
Active negation technique used newer SCSI-2 SCSI-3 drivers single-ended SCSI buses insure clean monotonic transitions signal. Drivers employing active negation contain ability only sink current source current well. These drivers usually "yank" SCSI signal negated state (i.e. high) actively pulling signal high with driver capable sourcing current. SCSI-3 Parallel Interface specification states: Additional benefit achieved using activenegation drivers DATA parity signals when operating fast synchronous data transfer mode reducing skews between first group signals (ACK, REQ, ACKQ, REQQ) DATA parity signals."
Must Terminators able Handle Active Negation?
When driver actively negates SCSI signal, pulled higher than termination voltage present active terminators. this occurs, current will begin flowing into regulated 2.85V termination voltage. active terminator cannot sink current that being sourced driver, then will begin raise above nominal 2.85V termination voltage which will result current greater than being sourced other SCSI signals being shared active terminator (which violates SCSI specifications). Since drivers placed point along SCSI terminators only exist physical bus, active terminators must able fully sink current generated active-negation drivers.
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Much Current Must Active Terminator Sink?
SCSI-3 Parallel Interface specifications allows sourced current driver line. nine" termination schemes like DS2107A, this means terminator must able sink lines line). DS2107A sink least which within this limit.
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