From: markw@airgun.wg.waii.com (Mark Whetzel)
Newsgroups: comp.sys.ibm.pc.rt
Subject: Re: What is differential SCSI?
Summary: Let me explain RT scsi and most likely what you have.
Date: 17 Nov 92 22:46:41 GMT
Organization: Western Geophysical, Div. of Western Atlas Int'l, Houston, TX

In article <1992Nov16.095729.23423@cs.hw.ac.uk>, censjg@caledonias3.caledonia.hw.ac.uk (Simon Greaves) writes:

   In article <36@antimatr.hou.tx.us> (Mark Whetzel) ... I wrote from
    my machine at home:

> |> You can run NON-IBM SCSI disks on the IBM SCSI adapter.  As long as
> |> they are either native differential, or run thru a Rancho diff-to-single
> |> converter box.  
> 
> Is this convertor a two way thing?

Yes the converter is BI-DIRECTIONAL...  It HAS to be... 
There are several companies that manufacture the DIFF <--> SINGLE converters,
in addition to Rancho Technology, but that vendor makes the ones that I have
personally used.

The box is more complex than looks on the surface, as EACH pair of
differential signals and each single ended lead is BI-DIRECTIONAL.  It is
truely a PARALLEL BUS.  That is why it is called a SCSI BUS.  :-)
The converter must arbritate and decide which end is "speaking" and drive
the leads for the other side, and "listen" to the talking device.  Remember
in a SCSI bus, even though there is a "initatior" and a "target".  The 
target devices are intelligent.  They can be given a command by the host,
disconnect from the bus, perform the command, arbitrate for the bus, and
re-connect asyncronously and deliver status information.  Independently
of operations pending or ongoing on other devices on the same bus.
Some target devices can themselves become initiators, allowing a 
disk drive to back itself up to a tape drive.. At least in theory...
I don't think that anybody has MADE one that does that... :-)
You can also have TWO hosts attached to the same bus, sharing a
common device.  HOWEVER.. Both host adapters would need to 
co-ordinate access to the device, and beware of hidden terminators on
most HOST interface boards.  Specs indicate that terminators should only
be on EACH end of the bus.  Also the host adapter in the other computer
must be changed to a different unique SCSI address  (range from 0 thru 7),
than either the attached device or other host computer.

The IBM RT/PC SCSI board from IBM is designed to DIFFERENTIAL SCSI specs.
There are two wires for each signal lead, when one goes hi, the other goes
low, in a push-pull arangement.
Differential SCSI on shielded cable will allow up to 25 Meters total cable
length.  Single ended cable length total bus length is 6 meters.
Both bus types allow for up to 8 devices (initatiors OR targets) on one bus,
provided that the total cable length is not exceded.

Most PC and workstation vendors sell SINGLE ended SCSI devices.  Single-ended
devices have only ONE lead is used per signal line.  The interface is
slightly simpler electronicly is the primary reason, and most devices are
usually "under the covers" of the host machine anyway, and don't need the
extra cable length or noise immunity that differential scsi provides.

SO you could end up with this:

 +----+
 |    |              +--------+                   +------+   +------+ 
 | RT |  DIFF SCSI   |        |   single-ended    |      |   |      |
 |    |--------------+  CONV  | ------------------+ DEV1 +---+ DEV2 +-- T
 |    |  up to       |        |   total length    |      |   |      |
 |    |  25 meters   +--------+   6 meters to     +------+   +------+ 
 +----+                           the terminator.

T = Terminator 

> I have a disk tray that has ~1.2Gb of disk on a differential SCSI and
> also has a Rancho RT-SDA in it. This is intended for connection to an RT,
   (the RT in the name is not just for the IBM RT.  It stands for
    Rancho Technology Single-Differential Adapter .. RT-SDA.. 
    The maker of the converter device... :-)
> so the intended "host" would be a differential SCSI device. I'd like to
> be able to connect this to eg a Sun or a DEC with single-ended SCSI,
> so can I attach the "host" machine (ie a Sun's single-ended SCSI) to
> the Rancho converter to derive differential SCSI?

Most likely the devices IN the tray and the disk drives ARE already 
single ended.  The converter just allowed connection of the differential
RT to the single ended devices.

NOTE:  The converter has no concept of IN or OUT.  It can have a 
single ended host, and differential devices just as easily.  

You can also  do this:

   Single                                                         single
   ended  <-- *N1 --> CONV <--- differential -- *N2 ---> CONV <-- ended --T
   host                                                           device
                                                                  *N3
*N1 - There could be single ended device(s) here..  6 Meters..
*N2 - And differential device(s) attached here...  25 Meters
*N3 - and of course single ended device(s) here..   6 Meters.
T   - Terminator.  The ranco converters have built in terminators for
      the differential AND the single-ended buses.
As long as there are no more that 8 total scsi controller devices.

The EXTREME example above actually has THREE buses.  Two single ended,
and one differential!
For a total cable length of 37 meters!  Mind you, this is pushing all
devices to their maximum specs, and there could be timing problems.
Especially if you had devices attached at all the points indicated. 
AND if especially fast devices are nvolved.
The converter box DOES introduce some delay when switching bus directions. 
Devices attached at the N1 and N2 spots on the buses shown in the last
example, MUST have no internal termination installed.. Usually a small resistor DIP or SIP pack, sometimes termination be enabled/disabled by jumpers.

Later,
markw
-- 
Mark Whetzel     My comments are my own, not my company's.
Western Geophysical - A division of Western Atlas International,
A Litton/Dresser Company           DOMAIN addr: markw@airgun.wg.waii.com
VOICE: (713) 963-2544		   UUNET address:  uunet!airgun!markw