In article <cabr82slqgyazc8vw6mb5_xz_hczayojjboespxt7dm-0a-n...@mail.gmail.com>,
Mattis Lind <mattisl...@gmail.com> writes:
> The VT125 coprocessor used an 8085 processor. Somehow it intercepted the
> serial line passsing through that strange white connector on the VT100
> board and processed the ReGIS commands.
The VT100 architecture allows expansion boards to intercept the host
communication stream by plugging into the "standard terminal port"
(STP) connector. This routes signals from the RS-232 (EIA) connector
on the back of the terminal through the STP and then into the UART on
the terminal controller board. This lets an add-on device intercept
ESC sequences from the host for processing by the add-on device before
they are sent to the VT100. The add-on device can also generate
responses to the host in the same way.
You can read more about this in Chaper 7 of the VT100 Technical Manual
<http://manx-docs.org/details.php/1,4071>
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