Accredited Standards Committee X3T9.2/91-86 X3, INFORMATION PROCESSING SYSTEMS* Document Number: X3T9/IR91/009 Project: Various Reference Document: RS-485 and ISO 8482 Date: 6/14/91 Reply to: Gene E. Milligan Seagate Technology Inc. OKM 251 P.O. Box 12313 Oklahoma City, OK 73157-2313 (405) 324-3070 FAX: 324-3794 VOICE MAIL: (408) 439-2800 -3- x 4370 (800) 553-3557 -3- x 4370 Fred Lucas General Datacom Inc. Chairman TR30.2 DTE/DCE Interfaces P.O. Box 1299 Middleberry, CN 06762-1299 Subject: International Version of RS-485 ASC X3T9 has generated several standards which reference Electrical Characteristics of Generators and Receivers for use in Balanced Digital Multipoint Systems EIA RS-485. In particular we are concerned with the IPI and SCSI interface standards. When earlier versions of the IPI and SCSI standards were published by ISO, the EIA RS-485 reference was changed to ISO 8482. This change was made on the basis that ISO required international references and with the understanding that ISO 8482 was the international equivalent of RS-485. Some semiconductor manufacturers have advised us that the requirements of ISO 8482 have been increased in certain key areas such as common-mode voltage extremes. They claim that because of the increased requirements differential transceivers can not be economically produced which comply with ISO 8482 and the speed requirements of IPI and SCSI-2 Fast. I am aware that one manufacturer is planning to announce a part which complies with ISO 8482 and SCSI-2 Fast. However they also will not be able to produce IPI Enhanced parts which comply to ISO 8482. In addition to the speed requirements, the transceivers are required for drives which are being implemented in decreasing form factors. There is a need for transceivers in high density packages. The semiconductor manufacturers indicate that a common mode voltage somewhat reduced from RS-485 would enable a significant increase in packaging density. *Operating under the procedures of The American National Standards Institute. X3 Secretariat: Computer and Business Equipment Manufacturers Association Tel: (202) 737-8888 311 First Street, N.W., Suite 500, Washington, DC 20001-2178 Fax: (202) 638-4922 X3T9 requests that you address the need for an additional alternative in an international standard providing the electrical requirements for differential transceivers. The additional alternative should meet the speed requirements of SCSI-2 Fast and have reduced common-mode voltage limits. Either that alternative or a second alternative should meet the speed requirements of Enhanced IPI. The pertinent requirements of SCSI-2 Fast, X3T9.2/86-109, 5/1/91, are: Low-level output voltage = 1.7V minimum at 55 ma High-level output voltage = 2.7V minimum at -55 ma Differential output voltage = 1.0V minimum with common-mode TBD (TBD is currently -7V to +12V but something in the order of -2V to +10V or less is desired) Input current per pin = +/- 2.0 ma max Maximum input capacitance = 25 pF Hysteresis = 35mV minimum Assertion = 30 nanoseconds max Negation = 30 nanoseconds max Period = 100 nanoseconds The pertinent requirements of Enhanced IPI, ISO 9318-6, 2/10/90, are: driver differential skew = 4 nanoseconds max driver differential transition timing = 18 nanoseconds max receiver differential skew = 5 nanoseconds max period = 40 nanoseconds We have attempted to get more information on what maximum common-mode voltages should be expected but have not gotten any useful data. The applications are all multi-dropped connected either by flat ribbon or twisted shielded pair cables. For SCSI-2 the maximum cable length including all drops is 25 meters with a cable which has 0.23 ohms maximum d.c. resistance per meter per line. SCSI-2 has 7 ground lines. For Enhanced IPI the maximum length is 50 meters with 28 AWG or 60 meters with 26 AWG. (The 40 nanosecond period applies at 10 meters. At 60 meters the period is 79 nanoseconds.) IPI has two ground wires. We would appreciate your effort to initiate a standards project for these additional alternatives. We would be delighted to learn that appropriate alternatives already exist in some international standard. Please let me know if you have any questions concerning this request or if we can offer any additional contributions as you proceed. As the project is initiated, assuming a project is required to fulfill the requirements, please provide me liaison information. Thanks very much for your help, G.E. Milligan X3T9 International Representative cc: Dick Brandt Chair TIA TR30 Del Shoemaker Chair X3T9 Bud Emmons Chair X3S3 Don Tolmie Chair X3T9.3 (IPI) John Lohmeyer Chair X3T9.2 (SCSI) Jack Wheeler IR X3S3