Modern external storage emulating RX02 (was Re: Most used toys, was Re: The late, great TRS-80)

[Ethan Dicks]

On 6/27/07, Allison <ajp166 at bellatlantic.net> wrote:
> Using a Tu58 on an -8 is not a goot match and there are lots of gyrations.

Agreed.

> >If all PDP-8s had a spare serial port, it might make sense to have a
> >serially-attached modern mass storage peripheral.
>
> Adding a second serial is trivial as there were many differnt serial
> cards available.  To make a TTY card RS232/432 passable is also not hard.

It's trivial from the OMNIBUS days onward to add another serial port.
Not so trivial for a Straight-8, an -8/S, an -8/L or -8/i.  One may
argue that machines that old don't matter so much, but I do happen to
have a BM08 on one --8/L (total of 12K) and would like to be able to
bring up OS/8 on it someday.  I'd also like to expand my -8/i to have
8K of core (and perhaps a full 32K eventually) and bring OS/8 up on
that as well.

Changing out 20mA for RS-232 isn't hard at all with the older machines
- Vince Slyngstad made some EIA paddle cards for pre-OMNIBUS boxes.  I
have at least two of his cards.  Some day, I'll find the time to
assemble them, but for now, I'm fine with hanging a VT220 off of my
-8/L with a 20mA cable.

> What is possible now is a small micro and a big static ram of 512k are
> which fairly easy to find it's not unreasonable to simulate a RX02
> using a micro at the end of a serial line (or parallel) and NOT use
> the protocal of TU58.

Sure.  There's no requirement to use the TU58 protocol, it's just
understood, is out there, and happens to work with a real device.  If
you are going to write an OS/8 driver anyway, there's no reason to
stick with a protocol that's hard to use.  I just dredged up an old
thread in my reading where someone suggested the TU-58 as the
"obvious" device for a diskless PDP-8.  I was just heading that debate
off at the pass, since it was extensively investigated over 20 years
ago and determined to be difficult, technically.

> The cpu/micro used does not have to be very high
> powered or fast as all it's doing is data transfer and PDP-8 PIO is
> usually slower than 30-40K words/sec.

Certainly not if you are rolling your own interface.  If you are
trying to make a plug-compatible RX02 emulator, there might be some
bit-level stuff that's timing critical, but the overall bandwidth is
rather low by modern standards.

> In the end what is used is more a matter of convenince than technology.

Agreed.

> I happen to be lucky(?) as my 8f has two serial cards but nothing
> else device wise. One of th cards is the usual console TTY but the
> other is a UART based M8652 that were often used for modem
> banks and serial data concentrators/switches made using PDP-8s.

Nice.

In amongst all the other recent PDP-8 discussions, I have to wonder
that if one was going to be spending $$$ on a 1 sq ft. PCB with edge
fingers and whatever line drivers, what would be a good choice of
peripheral options to stack on the same board.  For example, the
DKC8AA has several independent devices on one hex-height card.  In a
quad-height form factor, one could easily stuff two RX8Es, and at
least one, if not two KL8Es, which should take care of a lot of
external I/O requirements.  The RX8Es would use the standard OS/8
driver, of course, simplifying that aspect of things, but then one
could attach that to either a real RX02 if you had one, with floppies
to read/write, or to an off-board RX02 emulator as we've been
discussing.  Personally, I don't have even one RX8E per OMNIBUS
machine, so alternatives are an interesting direction for me.


-ethan