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

Allison ajp166 at
Wed Jun 27 17:47:21 CDT 2007

>Subject: Re: Modern external storage emulating RX02 (was Re: Most used toys,was Re: The late, great TRS-80)
>   From: "Ethan Dicks" <ethan.dicks at>
>   Date: Wed, 27 Jun 2007 16:02:14 -0400
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at>
>On 6/27/07, Allison <ajp166 at> wrote:
>> Using a Tu58 on an -8 is not a goot match and there are lots of gyrations.
>> >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.

If you have a parallel port or even an interface to read/write one bit
with an IOT (less parts than parallelport or tty interface) there are 
possible interfaces even for 8s.

>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.

20ma works too.  Though it's not hard to pick up the TTL or logic 
before 20ma conversion.

>> 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 need to buffer the tape data is the annoying part as well.

>> 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.

RX02 interface [RX8E] is fairly simple bit serial with clock.

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

What I  have is some Q or Ubus quad wide proto cards that could easily 
be used on Omnibus with a few cuts.  Parallel IO is spelled out in the 
interface handbook.

>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

DCKAA, have to look that one up.

>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.

If the driver were developed for the device it could be anything.
For example the device could be two parallel output and one input
port. One output port sets the block address (128word block for 
512kW) and the second is read/write to ram data with auto increment
to the low block counter.  That would be PIO, no micro and two 
512k byte wide rams (4bits wasted) and 5 74LS161s as the ram 
address counter (modulus 128). There are existing pdp8 parallel
IO cards that can do that.

When it cools down and I get a few minutes that might be an easy 
board to try and build. I imagine patching OS/8 is not hard as
the device page is locatable.  Though the complete package would 
need a test program, formatter and a device driver for OS/8.



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