Subject: Re: Rainbow & DECmate

From: [email-masked] (Charles Lasner)
Date: 20 Aug 1993 14:26:53 GMT

In article <[email-masked]> [email-masked] (N.E. Cole) writes:

>I also have a chance to get a DECmate (no number after it), what is this? I have
>the specs for a DECmate III+ but obviously I suspect it won't be quite as good as
>that! Is it possible to fit a HD, upgrade memory etc..???

For more specific info, post to alt.sys.pdp8.

Other than for a few specific options, DECmates are not upgradable in the
sense that you usually reference:

DECmate {no designator} means what is now referred in retrospect as DECmate I.
The system is 32K, and is the 6120 micro implementation of the venerable PDP-8
where 32K is the normal limit for all of the usual software.  Just enough of
the alternate control memory of 32K is also implemented to handle the 
peripherals, but only to the extent of implementing overall control/status,
and booting options, and a built-in terminal emulator only if the comm board
is present.  Thus, the only options are whether there are one or two pairs
of RX01/02 (I don't think it was sold with less than RX02, but it can take
RX01 or a pair of 01 and a pair of 02 or two pairs 02), the DP278{A or B,
where B supports bit-stuffing protocols and is an upgrade later for A} dual
comm board, and the RL-278 disk controller for 1-4 RL02's which of course
puts this VT-100-like box next to a very large cabinet or two.  A printer
port is built-in

DECmate II is by far the most flexible of the family, and consists of a
repackaged machine which is the same overall case as the PRO-325 and Rainbow.
Internally an RX50 is an option technically, but is always present.  Options
include an additional pair of RX50, the RX78 external RX01/02 board {which can
control one or two pairs of mixed RX01 or RX02 drives, neither of which can
be booted to directly, and are programmed in a slightly incompatible manner
to all previous designs which were totally compatible with each other!}, a
graphics board, possibly with the VR-241 DEC color monitor {yes, a second
monitor; there is no official one-color-monitor version, but the graphics
can be directed at the non-optional VR-201 mono monitor}, and a HD controller
which can connect one MFM hard disk.  {Note that the hard disk controller
precludes the RX78 option entirely, and usually the hard disk itself is
mounted within the case which precludes the second floppy pair.  While in
theory you could cable up an external hard disk, thus allowing a second RX50
pair at the same time, some of the software cannot be configured to support
the second RX50 pair and HD at the same time.}  The HD size can be up to
a formatted 64 MB since all DEC HD controllers of this vintage reserve
dedicated alternate sectors at the beginning of the disk (which is required
to be error-free in that area) and each track has 16 sectors of 512 bytes,
a maximum of 8 (not 16!) heads.  There is some debate over whether disks
beyond 1024 cylinders are supported, but in any case they are rare in the
industry anyway.  (For example, I use an ST-4096 which is 1024 cylinders,
but actually has 9 heads, one of which is wasted, but the 1024 cylinders
are otherwise totally used.)  There was a case upgrade for all of these
machines so that larger hard disks wouldn't overburden the power supply due
to the large start-up current of many hard disks, so you can't just drop in
a hard disk without checking the power supply ECO!  The only other option
for the DECmate II is a CPU and memory board that adds either a Z80 and 64K
or a Z80, 8086, and 256K {or alternatively 512K, not an upgrade, an alternate
board!} where the Z80 can access 64K of the memory only.  The 6120 is only
indirectly interfaced to this board, and the usual software environment is
that the 6120 runs as a server for whatever is running in the APU or XPU 
board itself when they are active.  The only exception is that a portion of
DEC's WPS software actually is run in the Z80 while the 6120 waits for the
results (DECspell, footnote formatting).  All other software runs without the
optional CPU and memory boards in 32K only, however the control-panel memory
is totally implemented to 32K.  However, virtually all of it is dedicated to
functions associated with the built-in terminal emulator, etc.

The software environment on a DECmate II disk is such that each application
owns its own volume or volumes, and can be sub-booted from Master Menu.  This
is necessary due to the myriad number of O/S'es that can run on the machine
which can have up to three CPU's.  {Some applications can be called up as
separate bootable entities, but most of these are actually bootable OS/278
systems that are dedicated to running a specific application program.  In
theory, it's possible to revamp them into manually invocable utilities called
up from one copy of OS/278, but this isn't what's done normally.  In fact,
Master Menu itself is one of these!  However, there are legitimately several
families of O/S for the machine that are distinct, and Master Menu can
accommodate all of them, such as WPS, COS, CP/M-80, MS-DOS, OS/278, etc.}

The DECmate III is a reworked smaller version of the DECmate II which can
only have one pair of RX50 and no hard disk.  The only options are a reworked
APU board with a Z80 and 64K, and a graphics board.  This system must have one
monitor but it could be a VR-241 color monitor as well as the standard VR-201
mono monitor.  All systems from the DECmate II up use the LK-201 keyboard,
usually sold with the WPS keycaps, which in turn implies the blue color scheme 
for the keyboard as opposed to the more likely red, etc.  In theory, there is
also an obscure modem option whereby some specially adapted DEC "scholar"
modem of the era can be plugged into a dedicated connector on the motherboard.
I've never heard of anyone ever seeing the actual board, but it's known that
it would be compatible with an appropriate DF modem that was 300 baud and 1200
baud only.  Some newer DECmate III cases even lack the little slide-out case
section where the modem's modular phone connector would poke through, or even
the little telephone symbol that is used to label the connector.  Early cases
come with a 78W power supply, but newer ones are rated at 150W.  In a 
startling display of retro-engineering, this system uses mono-voltage power
supplies, which means that, including the DECmate III+ supplies, DEC bothered
to reinvent so many wheels that there are at least six different power supplies
to be found in this box!

The DECmate III+ superficially looks like a DECmate III, but is quite different
inside.  The main board now sports an optional hard disk controller on an
plug-in daughter board, indicating the likelihood that the system was designed
to be a replacement of sorts for the original DECmate III board, but in fact
it was never marketed that way.  This hard disk controller is compatible with
the one from the DECmate II, but due to case restrictions, mounting an
internal hard disk requires it to be half-height; only the ST-225 20 MB disk
was ever sold with it.  The built-in modem connection of the DECmate III is
still present, but they don't even implement the chips to drive the connector!
The floppy support has been revamped to use RX33 (TEAC HD) drives, but only
mimicking the RX50 at 300 RPM.  It should be noted that the DECmate III+ can
only use RX33, and the DECmate II and DECmate III can only use RX50, since
the earlier machines require device-ready drives, and the DECmate III+
requires device-change drives.  (Although the TEAC drives can be strapped to
signal ready instead of change, they are not seek-ganged as is the RX50!)
The DECmate III+ supports two RX33 drives, but due to case limitations, they
were marketed with only one drive bolted onto the ST-225 HD.  It appears that
a contingency plan could have been to sell revamped DECmate III units by
deleting the hard disk controller daughter board and adding a second RX33,
but this configuration was never sold.  The same two APU and graphics option
boards of the DECmate III can also be used in the DECmate III+ however, CP/M-80
will have problems getting implemented, since it relies on the present of the
second floppy drive, even when getting installed.  (To install CP/M-80 on
the machine, you have to at least temporarily hook up an additional floppy
drive.  To do this requires an alternate floppy cable, although the wiring
is totally straight-forward and industry compatible.  Note that an IBM-PC
or PC-clone is *not* a straight-through cable and cannot be used!)

There is rumor that there was some form of XPU board for the DECmate III and
DECmate III+, possibly with more than the 8086 w/256K or 512K of the DECmate
II XPU, rumored to contain an 80286, but none have surfaced, and it certainly
wasn't sold.

It is known that the RX33 floppy implementation is slightly deficient, in that
the RX50 versions are capable of reading-*only* one-sided IBM-PC 40-track
disks, while the RX33 cannot.  Thus, it's not possible to write a utility
to convert from IBM-PC 160K or 180K diskettes.  On the DECmate II with XPU,
this is a standard feature of MS-DOS to read one-sided IBM diskettes, etc.

DECmate III+ comes with a 160W power supply.  The only seeming difference is
that there is an additional 4-prong power connector ganged into the power
wiring, as opposed to potentially different innards from the DECmate III
supply.  As in the DECmate III boxes, the supplies come in 110V and 220V
variants, instead of a switchable version as most equipment usually is
constructed.

cjl