PDP-11: DR11-C to FPGA or 1284?

Don North north at alum.mit.edu
Mon Jul 31 17:31:59 CDT 2017

On 7/31/2017 2:52 PM, Ethan Dicks via cctalk wrote:
> On Mon, Jul 31, 2017 at 1:52 PM, Fritz Mueller via cctalk
> <cctalk at classiccmp.org> wrote:
>>> On Jul 31, 2017, at 8:19 AM, Jay Jaeger <cube1 at charter.net> wrote:
>>> I have Ethernet shield for my Arduino Uno, and I use that and a simple
>>> (in my case, perl,  program to talk to the final destination device.  I
>>> have two cables, one for each direction, from the DR11-C (not using DMA)
>>> to the Arduino.
>> Jay, does your Arduino support TTL-level signaling, or did you have to use some level-shifting chips?
> "Arduino" covers a lot of hardware, but in the case of the Uno and
> Mega (and many other models), the GPIO pins are CMOS-level (5V Vcc,
> but CMOS thresholds, not TTL).  Not a massive fanout, but a few mA per
> pin - at least one TTL load.  You can't drive 8 LEDs from one port,
> but you can sink 1-2 LEDs on the same port (the specifics are well
> covered in the Atmel datasheets for each processor).
>> I'm more familiar with FPGA platforms than Arduino, but this might give me a good excuse to finally play around with Arduino a bit!
> I know there are people here who are not fond of them for various
> reasons (some non-technical), but I do a lot of quick-and-dirty things
> with them.  They are frequently overkill, but with clones at $6 (and
> licensed ones under $30), they do a lot.  Where you can run into
> complications is trying to use all the Arduino library function calls
> to, say, read and write I/O pins at megahertz speeds (the MCU is
> routinely clocke at 16MHz or 20MHz, and mostly
> one-instruction-per-cycle).  digitalWrite() and digitalRead() end up
> executing hundreds, if not thousand of machine cycles.  You can, of
> course, write in AVR assembler, but mostly, just banging on the
> relevant DDR and Data registers in C works plenty fast enough, much,
> much faster than the Arduino library calls.
> TL;DR - the chips are fine.  The libraries are heavy.  Someone coming
> from another architecture can get up to speed pretty quickly to read
> and write bits and bytes from GPIO pins.
> -ethan
Totally agree with the above comments. As an example, my RX02 drive emulator 
(runs on an Arduino Mega2560 with a custom interface shield) code is available 
here: https://github.com/AK6DN/rx02_emulator  as an example. Uses the SDcard 
library, and does very high speed bit banging on GPIO ports thru optimized C 
routines to act as an RX02 drive connected to RX11/RX211/RXV11/RXV21/RX8E-28 
interfaces in a PDP system.

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