[Simh] Additional Interfaces for the VAX780 simulator

[Ethan Dicks]

On Tue, Apr 14, 2009 at 5:38 PM, Gregg Levine <gregg.drwho8 at gmail.com> wrote:
> On Tue, Apr 14, 2009 at 9:36 AM, Ethan Dicks <ethan.dicks at gmail.com> wrote:
>> On 4/14/09, Hölscher <hoelscher-kirchbrak at freenet.de> wrote:
>>> I would like to have two (or more) additional interfaces for the VAX780 simulator:
>>>
>>>  DMC-11 (for DDCMP - DECnet, would be useful for the PDP-11 as well)
>>
>>>  Anyone else interested in those devices, or am I the only one?
>
> Hello!
> Well actually I am, Ethan. Especially the DDCMP connection.

That question was asked by Hölscher, not me.

> I am still curious if the connections posing as a serial one can be
> connected to a physical device. I believe the Altair/Imsai one can be
> connected to one such device, but I've also never had a reason to try
> it.

What I said in response is still correct - "standard" PC hardware has
no way to speak sync serial - there are no start and stop bits, among
other things.  The closest I suppose you could is to attempt to
bit-bang out serial frames via a parallel port, but then you'd still
need a level shifter from TTL to EIA levels.

A UART is not USART, though some chips, like the Z8530, can be used in
either mode.  The 8250 (and its descendants the 16450, the 16550, and
more recent implementations that are a corner of a motherboard
chipset) just can't emit the right bitstream to connect up to a
classic sync serial device.

<http://www.freebsd.org/doc/en/articles/serial-uart/index.html>

One could whip up a virtual USART that communicated over a network
socket or some other IPC mechanism to another instance of simh, but
not to a real machine via a real connection, not without adding
hardware to the machine that was running simh.

If you are willing to run simh on a PC that's old enough to still have
ISA slots, it's possible to find (or semi-easily build) a serial board
that could do the job, given the right drivers and new code in simh.
It's not as simple as just writing new device emulation code in simh.
One could build a sync-serial interface on a PCI card, but that's
nowhere near as easy as building an ISA card.

-ethan