Fast-20 and differential cable lengths

Lohmeyer, John JLOHMEYE at
Thu Nov 2 11:01:00 PST 1995

Ernie wrote:
>In the draft fast-20 spec, the differential cable length is stated as
>max 25 meters.  I've heard some concern that this has been drastically
>reduced,  The recent discussion about fast-20 cable lengths i'm assuming
>was for single-ended implementations.
>Could someone state the spec on fast-20 *differential* cable length 

The Fast-20 draft standard permits 25 meters of total bus length for 
differential Fast-20 systems.  The cable length reductions were for single 
ended systems:

Driver/receiver type  Max cable length  Number of devices

differential          25  meters        8 or 16 (bus width)
single-ended          1.5 meters        8 devices
single-ended          3.0 meters        4 devices *

* devices must be spaced evenly

These cable length reductions on single ended are due to the marginal 
driving capabilities of single ended.  The X3T10 committee is now working on 
developing a Low Voltage Differential Signaling (LVDS) that will cost 
slightly more than single ended and have MUCH better signal quality. 
 Tentative specs:

Driver/receiver type  Max cable length  Number of devices

LVDS                  12 meters (+)     16

Furthermore, LVDS will support Fast-40 speeds and very likely can be 
extended to well beyond Fast-80 speeds.  The key is reducing power enough to 
permit integration of the LVDS drivers and receivers on the protocol chip.

By the (+), I mean that even longer cables can be used with fewer devices. 
 Early testing suggests that 25-35 meters of cable length is feasible in 
point-to-point applications.


John Lohmeyer             E-Mail:  john.lohmeyer at
Symbios Logic Inc.         Voice:  719-573-3362
1635 Aeroplaza Dr.           Fax:  719-573-3037
Colo Spgs, CO 80916     SCSI BBS:  719-574-0424 300--14400 baud

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