[Coco] SLENB line behavior

[RETRO Innovations]

On 9/20/2016 3:02 AM, Phill Harvey-Smith wrote:
> On 20/09/2016 07:46, Barry Nelson wrote:
>> I am not sure that the system would determine that, as SLENB is a
>> input. Wouldn't wither multiple devices could share SLENB depend on
>> those device's output circuits? In other words, if I attach a device
>> to the system that forces SLENB high with a low impedance, then no
>> other device could pull it low.
>
> Indeed SLENB should only ever be pulled low, it should be allowed to 
> float high (as it's pulled high inside the machine). Ideally it should 
> be an open collector driver or the equivalent if driving it with a PAL 
> / CPLD / FPGA.
>
> Something like (in Verilog) :
>
> SLENB = enable ? 1'b0 : 1'bz;
Which is how I have my Verilog enabled.  But, with the other cart on the 
MPI, I can't seem to drag the line low.  I even used the following 
equation to try to drag it low:

assign SLENB = !E;

(realizing the Coco would probably not boot, but I figured I should see 
the inverse of E on the SLENB line).  With the other cart onboard, i get 
a few 62.5ns blips where the line goes low, but nothing else.  Without 
it on the bus, I see the expected behavior. I taped over the SLENB line 
on the other cart and the issue disappears, so I am confident I have 
found the source, but didn't know if it was expected behavior or not.

I started to send a question about the cart, but then thought maybe my 
understanding of the line was faulty.

Jim