Doc: X3T9.2/87-160 Revision 0 September 2, 1987 To: X3T9.2 Committee From: Jeff Stai, Western Digital Corp. Much time was spent at the working group discussing the pros and cons of various wide scsi cabling schemes. This document attempts to relate the thinking of the working group (which contained many opinionated people on all sides of the issues), and present the results. I hope to convey an impression of the process that we went through in an attempt to reduce the amount of debate necessary to arrive at a satisfactory solution for all. GOALS ----- The working group (nearly unanimously) felt that the following points were generally agreeable: (1) Downward compatibility to the eight bit bus. (2) A 'micro-connector' alternative was needed because of new small products (3.5" drives and MicroChannel boards, for example). (3) The differential option required a DIFFSENS signal on the second cable if a two cable option is adopted. ONE CABLE 8/16 -------------- This option puts 8 and 16 bit operation on one cable. The other 16 bits required for 32 bit operation would be on a second cable. This option would require a 68 pin cable for the A cable. Those in favor of a single cable for 16 bit operation felt that this was a good thing because one cable would reduce cost of cabling, save board space, and it had the potential for easier upgrade of 8 bit devices because only one REQ/ACK pair is required to handshake 16 bits. Those opposed pointed out that the upper and lower bytes will have different electrical lengths in mixed 8/16 busses; the 8 bit portion will have to be broken out at various points in the daisy chain; this causes greater skew between data lines. Also, the cables to convert 50 to 68 would have to be custom made. This option was finally rejected for the following reasons: - the electrical length issue would in fact require an additonal REQ/ACK pair to manage the skews associated with mixed busses. - a compromise was reached that was acceptable to those who did not want a two cable solution (see below). TWO CABLE 8/16/32 ----------------- This option puts both 16 and 32 bit operation on a second cable. The original 8 bit cable is retained unchanged. This option is what is currently in the SCSI draft. This option was accepted because of the electrical issues; there was no reasonable solution to this problem. One idea was to use two REQ/ACK pairs on the A cable (one for each byte on the cable) to manage the skews, with a third REQ/ACK pair on the B cable. This was not well liked... Some minor changes were accepted for the two cable proposal that made it easier to take by the one cable side. These were: (1) Reordering the B cable. The signals required for 16 bit only operation were moved to one 'side' of the connector. (2) The B cable must be a 'micro-connector' Another point: AMP informed us that their micro-connector will be built for surface mounting. We expect that other connector types will be able to provide this feature. This will allow mounting on both sides of the PC board (if that's what you want to do...) to save board space. ODD BYTE PROBLEM ---------------- The working group proposes the following solution to the wide SCSI odd byte problem: Two signals have been added to signal the number of bytes that are valid on the B cable. This solution was felt to be superior to the use of a signalling message after the data phase, since it occurs during the data phase when the word is transferred, and may occur easily at block boundaries. The signals are interpreted as follows: BVAL1 BVAL0 number of bytes valid on B cable: ----- ----- --------------------------------- 0 0 0 bytes 0 1 1 byte 1 0 2 bytes 1 1 3 bytes ----- ----- --------------------------------- DIFFERENTIAL SENSING -------------------- It was noted by the working group that the DIFFSENS signal which appears on the differential A cable does not appear on the B cable. The group decided that it should be added to the B cable. SIGNAL ORDERING --------------- It was noted by the working group that the REQ and ACK signals were adjacent on the connector. Since these are the fastest switching signals on the interface, it was felt that they should be separated further to prevent possible coupling between these signals. RECOMMENDATIONS --------------- The working group recommends to the plenary the following actions: (1) Formal acceptance of the current two cable concept for SCSI-2; (2) Formal rejection of the single cable 16 bit mode of operation for SCSI-2; (3) Formal acceptance of the new 68 pin B cable for 16 and 32 bit transfers, with the changes proposed by the working group; (4) The working group recommends that the plenary select one of two types of micro-connector: a pin-and-socket arrangement, and a 'ribbon' (I would call it a 'micro-Centronics') connector. A presentation will be made at the plenary on the pros and cons of each type. PROPOSAL -------- The new B cable proposal appears on the following pages... Table 4-2: Single-Ended Pin Assignments Cable B ============================================================================== Signal Pin Number ------------------------------------------------------------------------------ -DB(8) 2 -DB(9) 4 -DB(10) 6 -DB(11) 8 -DB(12) 10 -DB(13) 12 -DB(14) 14 -DB(15) 16 -DB(P1) 18 -ACK(B) 20 GROUND 22 -REQ(B) 24 GROUND 26 -BVAL0 28 -BVAL1 30 TERMPWRB 32 -DB(16) 34 -DB(17) 36 -DB(18) 38 -DB(19) 40 -DB(20) 42 -DB(21) 44 -DB(22) 46 -DB(23) 48 -DB(P2) 50 -DB(24) 52 -DB(25) 54 -DB(26) 56 -DB(27) 58 -DB(28) 60 -DB(29) 62 -DB(30) 64 -DB(31) 66 -DB(P3) 68 ============================================================================== NOTES: (1) All odd pins except pin 31 shall be connected to ground. Pin 31 shall be connected to TERMPWRB. (2) The minus sign next to the signals indicates active low. Table 4-4: Differential Pin Assignments Cable B ============================================================================== Signal Name Pin Number Signal Name ------------------------------------------------------------------------------ SHIELD GROUND 1 2 GROUND +DB(8) 3 4 -DB(8) +DB(9) 5 6 -DB(9) +DB(10) 7 8 -DB(10) +DB(11) 9 10 -DB(11) +DB(12) 11 12 -DB(12) +DB(13) 13 14 -DB(13) +DB(14) 15 16 -DB(14) +DB(15) 17 18 -DB(15) +DB(P1) 19 20 -DB(P1) +ACKB 21 22 -ACKB DIFFSENS 23 24 GROUND +REQB 25 26 -REQB +BVAL0 27 28 -BVAL0 +BVAL1 29 30 -BVAL1 TERMPWRB 31 32 TERMPWRB +DB(16) 33 34 -DB(16) +DB(17) 35 36 -DB(17) +DB(18) 37 38 -DB(18) +DB(19) 39 40 -DB(19) +DB(20) 41 42 -DB(20) +DB(21) 43 44 -DB(21) +DB(22) 45 46 -DB(22) +DB(23) 47 48 -DB(23) +DB(P2) 49 50 -DB(P2) +DB(24) 51 52 -DB(24) +DB(25) 53 54 -DB(25) +DB(26) 55 56 -DB(26) +DB(27) 57 58 -DB(27) +DB(28) 59 60 -DB(28) +DB(29) 61 62 -DB(29) +DB(30) 63 64 -DB(30) +DB(31) 65 66 -DB(31) +DB(P3) 67 68 -DB(P3) ============================================================================== NOTE: (1) SHIELD GROUND is optional. IMPLEMENTORS NOTE: Some shielded flat ribbon cables use pin 1 as a connection to the shield. [Add the following new paragraph:] 4.4.4 Cable B Valid Bytes The SCSI bus device driving the data bus DB(31-8) on Cable B shall also drive the bytes valid signals BVAL0 and BVAL1. These signals specify the number of valid bytes on Cable B during a data handshake. The BVAL1 signal shall not be asserted if a 16 bit data path has been established between the two devices. The BVAL1 and BVAL0 signals shall not be asserted if an 8 bit data path has been established between the two devices. The following table summarizes the interpretation of these signals. Table 4-x: Cable B Valid Bytes ============================================================================= BVAL1 BVAL0 Interpretation ----------------------------------------------------------------------------- negated negated no bytes valid on cable B negated asserted one byte valid on cable B asserted negated two bytes valid on cable B (32 bit data path) asserted asserted three bytes valid on cable B (32 bit data path) ============================================================================= [ I expect there are better words for this... This function cannot be defined except in terms of information supplied later in the document...]