Re: HP 50g Connection to Multimeter


I suppose that that depends on what you mean by "activity
detection". Mine certainly does detect whether the data receive
line is at a valid (mark or space) or an invalid (idle or
floating) level.

I apologize for my misunderstanding. Eric also pointed this out to me.
You do in fact use the term "active" consistently to mean a valid
signal as opposed to my understanding that an active signal has
activity i.e. it changes. So when I wrote that we don't do activity
detection, what I meant was the hpcalc cable doesn't shutdown after a
period (e.g. 30 seconds) of no signals changing (inactivity). This is
notable because Samson's cable does (or at least claims to, I've never
actually seen one). Also the MAX3227 implements this automatically and
refers to it as "AutoPowerdown-Plus".

For more information on how the current version of the "hpcalc" cable
functions, see http://www.allenwan.com/hpcalcserialcable.html

But how to detect whether the serial port is open or closed? It
seems to me that when the port is open, the transmit line must
always be driven to a mark (+3.3V) or space (0.0V) condition. When
the port is closed (including when the calculator is off), I'd
presume that this line would be driven to a space condition or
perhaps left floating. When open and nothing is being transmitted,
the transmit line should be driven to a mark condition (+3.3V)
(like a continuous stream of stop bits), and when something is
being transmitted, it must be driven to the mark condition at
least once (for a stop bit) per frame. So if it isn't driven to a
mark condition at least once per frame time at the lowest bit rate
(longest frame time), then the port must be closed. Any return to
a mark condition would indicate that the port has been opened
(even if not currently transmitting). Presumably this is what the
extra IC (the microcontroller) is for.

The most challenging aspect of the calculator's behavior to deal with
is that the receive line is driven low when the calculator is turned
off or the serial port is closed. For a more detailed explanation of
how this is handled, see http://www.allenwan.com/hpcalcserialcable.html

So, Allen, where do you get your information about this from?

I don't know. It just came to me. :-)

Do you actually have an adapter from commerce.hpcalc.org?

I don't. I haven't had any reason to order one since I can make one
myself for much less. :-)

-Allen
my email address is mm-dd-yyyy@xxxxxxxxxxxx where mm-dd-yyyy is
today's date.
.

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