Help me! Before I take the plunge

Wai-Sun Chia waisun.chia at
Thu Mar 31 22:17:27 CST 2005

On Apr 1, 2005 5:27 AM, Glen Goodwin <acme at> wrote:
> Okay, so out of curiosity I found the listing for the Decmate on eBay:
> Now, this is a pretty cool looking critter.

Yeah. It's a beautiful specimen of early 80s DEC engineering /

> Is it a fair value at $200 plus (probably) another $200 for shipping?
> Is technical documentation available?

Well that depends on who you're talking too.
These are I think the only 3 docs available online for the DECmate
(VT278) system.

> What OSes will run on it? (OS/8 ?) Are they available for download?

It runs OS278, different enough that AFAIK most OS8 software won't
run. The incompatibilities is mainly due to the non-standard
non-OMNIBUS hardware boards that it uses.

The below is a part of Doug Jone's superb FAQ on PDP8 and it's variants.

        A new feature was introduced in the 6120 microprocessor:
        The Group I OPR combination RAL RAR was defined as R3L, or rotate
        accumulator 3 places left, so that byte swap (BSW) is equivalent
        to R3L;R3L.  RTR RTL remained a no-op, as in the 6100.

        Also, the EAE operations not implemented in the basic CPU cause
        the CPU to hang awaiting completion of the operation by a
        coprocessor.  Unfortunately, no EAE coprocessor was ever offered.

        The printer port offered software baud-rate selection compatable
        with the VT78 baud-rate selection scheme.  The dual-port data
        communications option was flexible but completely incompatable
        with all previous PDP-8 serial ports.

        The console and printer ports are not fully compatable with the
        earlier PDP-8 serial ports.  Specifically, on earlier serial
        interfaces, it was possible to test flags without resetting them,
        but on the DECmate machines, testing the keyboard input flag
        always resets the flag as a side effect.  In addition, on the
        console port, every successful test of the flag must be followed
        by reading a character or the flag will never be set again.

        It was not possible to continue from a halt without restarting
        the machine.

        The large amount of device emulation performed by the CPU in
        supporting screen updates severely limits the ability of the
        system to run in real time.

Don't know whether OS278 is available or not or for which media. Have
to look for it by yourself.

> What software, languages, assemblers, etc. are available?
> How compatible is it with the PDP-8 -- ie, will it run PDP-8 software?
> Does it have a free serial port so I can download software onto a PC and
> then serial-transfer it to the Decmate?

Depends on whether it has the optional DP278 board which provides 2
serial ports.

> How expandable is it?  What sort of bus does it have?

Takes in 3 slots at the back (these are not regular OMNIBUS slots,
it's proprietary to the DECmate).
Slot 1: CPU board + floppy + printer  + keyboard
Slot 2: Optional (typically DP278 serial controller)
Slot 3: Optional (typically RL278 disk controller)

The DP278 and RL278 are quite rare nowadays. If your unit doesn't
already have them, you'd be hard pressed (at least  your wallet will)
to find them.

Another one of DECmate's advantage is that:
You get a VT100 free! :-)
[Caveat: This is only if you have a DP278 board. Otherwise you'd have
no where to connect it to the host...]

> What are the Decmate's disadvantages?
> Is there a better machine I should hold out for which is small and in
> the same price range?

I don't know, but to me:
- DECmate! Such character, and it's soooo 70s :-)
- DECmate 2.. eh? Looks like a PC AT?
- DECmate 3.. Is that a computer or an external disk unit?

> If it has problems, are there people on this list who will help me
> troubleshoot the thing (I have a 'scope, logic probe, meter, soldering
> tools, etc.)?

The prints/schematics are available.


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