[GreenKeys] IBM 2741

[John Nagle]

On 7/23/2011 7:18 PM, greenkeys-request at mailman.qth.net wrote:
> Now there are other kinds of terminals, such as the IBM 2741, which
> automatically lock the keyboard after you hit the ENTER key, and you
> can't send any more until the computer you are connected to sends a
> signal to unlock the keyboard.  I have a wry saying that in the older
> IBM world the only input device is a card reader, and it doesn't read
> a card until it is told to.  So they make a terminal behave like a card
> reader by not letting it send any input until the computer tells it to.

    No, that was because the device could be used in a multi-drop mode.
You press the ATTN key, and when it lights up and the keyboard unlocks,
you get to type, and nobody else on the line does.  When you press
ENTER, you're releasing the line, and someone else can use it.
Early inquiry systems worked that way.

    A similar arrangement was sometimes used for Teletypes.
Each station had a TD, which would turn on, if it had
tape to send, when sent the appropriate station poll.
Teletype 35ASR machines with a stunt box could be set up for this.
In the 1970s, this was used with a computer doing the polling,
and the Plan 55-A manual from the 19650s indicates that a Plan 55-A
center could poll Model 28 machines in a similar way.

				John Nagle