Insert a delay between the CPU writing a value to a periperal and the value actually being written to the periperal.

e.g. :
Code
void rmnimbus_state::fdc_reg_w(offs_t offset, uint8_t data)
{
	if((offset == 0) && ((data & 0xF0) == 0xD0))
		wait(64);

	m_fdc->write(offset,data);
}

Cheers.

Phill.