[Coco] SLENB line behavior

[Phill Harvey-Smith]

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;

It should also be noted that you also shouldn't use a pullup resistor on 
your cart on SLENB either as this (in parallel with the internal 
resistor) will make the line harder to pull down.

Cheers.

Phill.