How a HP-IL peripheral signals "no more data"

[J-F Garnier]

This is definitively not a question of "newbie" !

To keep it simple: generally the right way for a device to signal no (or no more) data available is to reply with EOT (End of Transmission).
Then, it's up to the controller to decide what to do: if the controller is happy with this reply, the transaction is over (the 41C acts in this way), or if the controller explicitly needs a end of data terminator such as CR/LF, the controller may retry with a new SDA (Send Data).

The HP71B can act in both ways: by default it will do SDA reties until a termination condition is met (CR/LF, END byte or number of bytes), but can also end the transaction like the 41C when an EOT is received, if the flag -23 is set.

Both ways have con and pro: in the one case, the transfer never hangs but may be incomplete and require a new explicit transfer retry; in the other case, the transfer will automatically continue until completion but may hang if the device never send the requested data.

J-F