On 2014-01-29 10:00, Kyle Owen<kylevowen at gmail.com> wrote:
>
>I don't remember ever seeing a Model 33 with parity. And weren't
>telegrams sent in 5 level code? If not at the end, then surely before
>ASCII appeared. And of course before either, there was Morse code, which
>doesn't come with parity either.
>
>All parity can do is convert garbled characters into missing characters.
> Neither is good. Telegraph operators probably relied on having good
>signal quality, ensuring adequate bit error rate values, at which point
>parity is not particularly needed. And with Morse, you're probably relying
>on skilled operators -- ECC performed by trained brain cells.
Wikipedia seems to indicate that the Model 33 ASR sent 7-bit ASCII with
even parity. I've found that in many of the programs on my PDP-8/E, even
parity is required, which I presume stems from the Model 33 ASR days.
Eh? What? There was an option for 8-bit clean communication in the
ASR33, as well as other parity choices. But by default an ASR33 have
*MARK* parity.
And all older PDP-8 software that I've ever seen also assumes MARK
parity. If you try running older PDP-8 software with your terminal set
to even parity, it will not work.
If you are ever near an ASR33 with a paper punch, you can easily verify
that it is doing mark parity by just enabling the punch, and check what
codes you get there as you type, with the ASR33 in local mode.
Johnny