Subject: PDP-8

From: [email-masked] (Douglas W. Jones,201H MLH,3193350740,3193382879)
Date: Thu, 24 Sep 1992 16:37:50 GMT

I have a PDP-8/E and a PDP-8/F at home, in various states of restoration.
For each, I have an RX01 drive, one that may have worked when turned off,
and one that needs a new drive transistor.  Does anyone know the proper
JDEC number for the stepping motor drive transistors in an RX01?

Also, I only have one RX8E interface board, so I can't use both drives
until I get another RX8E.  Does anyone know where that interface is to be
found?
					Doug Jones
					[email-masked]

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From: [email-masked] (Charles Lasner)
Date: Thu, 24 Sep 1992 22:22:58 GMT

>until I get another RX8E.  Does anyone know where that interface is to be

There are only a few handlers for using a second RX8E on one machine, but
it is supported by changing the device code selected for the board from 75 to
I believe 76.

There are other ways to get more than one pair on a machine:

The VT-78 supports two pair on one interface and uses the new instruction
SEL (6750) to select which pair.  Power-clear default is to select pair 0
which is the same pair used by the (ignorant of all of this) OS/8 system
handler.  When the non-system handler exits, it must ensure that the feeble
system handler addresses the pair 0 drives in all cases, including error 
returns.

The VT-278 supports the two-pair mode as well.  The DECmate II supports two
pair of RX50 and optionally one or two pair of RX01/02 as well on the 
RX78 option board.  If that board is used, a hard disk is not allowed as the
hard disk controller requires being plugged into the same slot.  The
select process is the same except that additionally SEL is responsible for
another bit that switches between RX50 and RX01/02.  (AC[11] is the pair bit
and AC[0] is the RX type switch.)  When the RX78 isn't present, only the
pair select is active.  Warning: there are certain RX01/02-specific programs
such as RTFLOP which can't run on the DECmate unless it's a version that
sets the RX-type bit.  Also, if the RX78 hardware isn't present, the program
will erroneously attempt access of RX50 disks, which are not supported.

There is also a non-DEC version: the DSD-210 supports four drive selects
on its drive buss.  In fact, there is a known variant where three drives are
installed vertically in one chassis.  By extending the buss, up to four drives
are allowed.  If you examine the command structure of the RX01/02, there is
a "vacant" bit to the left of the unit select bit, and DSD uses that as the
high-order select bit for access to other drive(s).

The P?S/8 system and non-system handler for RX01 actually work on all of
the described configurations, although a reassembly is required to change
the device code to support a second RX8E.  This is accomplished by always
setting both SEL bits accordingly, and also setting the DSD unit-select bits
as well.  In all versions, some aspect of this gets ignored, so there need
not be any special-case checks.

cjl