HP41 FI data line

[Nate Martin]

Should the FI data line go high or low to indicate data available? FI line from the I/O ports?

Thanks,

Nate

[Diego Diaz]

Hi Nate,

In short, no, FI is the Flag Input line.

I'll add that this is the obscurest and most poorly documented line from the I/O interface. :-)

It's used only by peripherals (PRINTER, HP-IL, WAND, etc) when requesting service from the CPU and is held high for a full digit time (four clock bit time cycles). The exact digit when the FI line is high informs the CPU of the type of service/periphera requesting service.

To the best of my knowledge, apart from said peripherals, only the CL CPU (for obvious reasons) and the USB-41 (to simulate 82143A printer) handle this line. It has no use on ROM pacs or any other module.

Cheers from Canary Islands.

Diego.