Thinking about PDP11 PC05 Emulation

[Brent Hilpert]

On 2019-Mar-11, at 2:37 PM, allison via cctech wrote:
> On 03/11/2019 02:11 PM, Jay Jaeger via cctech wrote:
>> I have several PDP-11's in my collection (among other things), and not
>> enough PC05 tape readers (or enough room) to go around.  But most if not
>> all of my machines have M7810 PC11 interfaces, and I have one I could
>> move from machine to machine as needed.  Moving a PC05 around would be a
>> lot more work, and not every rack has room.  ;)
>> 
>> So, I took a look at what it might take to interface with an M7810 (or,
>> down the road, a PDP-8/L or PDP-12.  It looks like the emulator would
>> have to accept as input just 3 lines (Initialize  L, IOP2(1)/Select,
>> IOP4(1)/Read) [It would not need the redundant Initialize H, IOP1(1),
>> Qualify or Skip], and would have to drive 11 lines into the pullups on
>> the M7810 (8 Data lines, IO Bus INT L/Reader Done L, Outtape/Error and
>> RDR RUN L/RDR Busy L).
>> 
>> So, a total of 14 interface lines. (The 8 or 12 would take a few more
>> lines).

. . . 

>> BUT - it also occurs to me someone may have already done something like
>> this?  Any leads / ideas?

. . .
> To do the data you need 8 bits but you can bit bang them out using two
> lines on a nano to
> a 74ls164.  The rest you use transistors (open collector) to do high
> current (though 5V,
> 1K pullup is only 5ma) and I'd do that to make the IO more rugged and
> ESD proof.  That
> covers the strobes and control lines.  Just using two lines to get the 8
> data lines via a 164
> frees enogh pins for there to be surplus IO lines.

. . .

I've used an RPi for tasks like this in much the same way as Allison is describing -
reduce the number of I/O pins needed on the modern microcontroller by serialising
the legacy-device parallel data lines with a simple TTL shift register.
2-4 pins (CLK,LATCH,DIN,DOUT, depending on app) from the microcontroller
can be translated to 8,16,32 or as many data lines as you need.