(HP71B) RAM access in HP71B; how it works?

[mfleming]

(04-09-2024 05:31 PM)KeithB Wrote:  HP already did:
https://literature.hpcalc.org/community/...rdware.pdf

Well yes, it's all there . . . but that's like saying the picture on the front of a jigsaw puzzle box is "all there" inside the box - once you put all the pieces in their proper place

There is quite a bit to learn how devices work in the 71B, most but not all of it explained in the IDS. I'll give you a high-level handwaving idea for a starting point. You can dig in from there.

• When the 71B wakes up, it probes each port in turn by raising and later lowering the port's individual Daisy In signal. This configuration process is done twice, first to identify what is in each port, and a second time to provide valid addresses for all devices in each port.
• The 71B then sends out an ID command. The device receiving the Daisy In signal will respond with a five nibble identification string that specifies what it is, how big it is, etc. If there is no response to the ID command then the port is empty or the end of a chain of devices has been reached.
• Once receiving a valid response, the 71B will send a CONFIG command with a five nibble address for the chip. On the first discovery pass, all devices are given the $40000 address. In the second configuration pass each device is placed in the address space (32KB and larger devices from the top down, smaller from the bottom up).
• If there are multiple devices in a given port then devices are daisy chained together, where each responding device passes the Daisy In signal to the next device in the chain once it has been given a configuration address.
  

Note that in the first discovery pass an extra ID command goes out, and a lack of response terminates the discovery phase for that port. Afterwards, the Daisy In signal for that port is lowered as the 71B goes to the next port.

In the second configuration phase, for a non-empty port the Daisy In signal is raised but no ID command is needed since the 71B knows what devices are in the port. A series of CONFIG commands are sent to put each device in their proper place in the address space.

J-F, please be sure to razz me for anything wrong I have wrong

~Mark