Note: cables longer than 6 meters require external differential transceivers which add delay and degrade the performance even more than indicated here.
Our simulations say that under very best conditions (fast silicon, low temperature, high voltage, zero length cable) we can expect more than 8 MB/s asynchronously. In the lab, I routinely measure 5 MB/s on 8 foot cables. So, if you were writing the data manual for this, how would YOU spec it?
The framers of the SCSI spec threw in synchronous mode to boost the performance on long cables. In synchronous mode, the sending device is permitted to send the next byte without receiving acknowledgment that the receiver actually received the last byte. Kind of a ship and pray method. The acknowledgment is required to come back sometime, but we just don't have to wait for it (handwave the offset stuff and the ending boundary conditions). In this mode any external transceivers add a time shift, but not a delay. So if you negotiate for 5 MB/s, you get 5MB/s regardless how long the cable is and regardless whether you are single-ended or differential. But you can't go faster than 5.5 MB/s, except in SCSI-2.
Synchronous mode does have a hold time (unlike asynch) but again, setup and hold times subtract out. In SCSI-1 synchronous mode, the speed limit comes from the combined ASSERTION PERIOD + NEGATION PERIOD which is 90ns + 90ns = 180ns = 5.5 MB/s. Our 53C90 family doesn't quite hit the max, but we do guarentee 5.0 MB/s. In SCSI-2, anything above 5.0 MB/s is considered to be FAST. Here the maximum transfer rate is explicitly limited to 100 ns or 10MB/s; you don't have to read between the lines to deduce it.
Interesting tid-bit: given a SCSI-2 FAST period of 100 ns and a cable delay of 131 ns on a 25 meter cable, you can actually stack 1.31 bytes in the 8-bit cable. In FAST and WIDE SCSI you can stack 5.24 bytes in this copper FIFO.
An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than using resistor voltage dividers.
This is a passive terminator:
TERMPWR ------/\/\/\/------+------/\/\/\/----- GND
Notice that the termination voltage is varies with the voltage on the TERMPWR line. One voltage divider (two resistors) is used for each SCSI signal.
An active terminator looks more like this (supply filter caps omitted):
TERMPWR -----| in out |------+------/\/\/\/-------SCSI signal
| gnd | |
| +------/\/\/\/-------SCSI signal
GND ---------------+ |
Assuming that the TERMPWR voltage doesn't drop below the desired termination voltage (plus the regulator's minimum drop), the SCSI signals will always be terminated to the correct voltage level.
The SCSI specification is available from:
Global Engineering Documents 15 Inverness Way East Englewood Co 80112-5704 (800) 854-7179 SCSI-1: X3.131-1986 SCSI-2: X3.131-199x SCSI-3 X3T9.2/91-010R4 Working Draft
Global Engineering Documentation in Irvine, CA (714)261-1455
SCSI-1: Doc # X3.131-1986 from ANSI, 1430 Broadway, NY, NY 10018
IN-DEPTH EXPLORATION OF SCSI can be obtained from:
Solution Technology Attn: SCSI Publications PO Box 104 Boulder Creek, CA 95006 (408)338-4285, FAX (408)338-4374
THE SCSI ENCYLOPEDIA and the SCSI BENCH REFERENCE can be obtained from:
ENDL Publishing 14426 Black Walnut Ct. Saratoga, CA 95090 (408)867-6642 FAX (408)867-2115
SCSI: UNDERSTANDING THE SMALL COMPUTER SYSTEM INTERFACE was published by Prentice-Hall, ISBN 0-13-796855-8