[Simh] Rainbow100

[Timothe Litt]

> And the VT240 was very slow. I never saw or used a VT125, so I don't know
> >/how it compared, but it didn't have color, right? /
>
> It did have color. You could connect an external RGB sync on green
> monitor to the BNC connectors at the back of the VT125.  For output on the
> built in black/white display it generated a grayscale signal that was
> mearly overlayed on top of a the VT100 signal. This also meant that on the
> RGB connectors you only had the VT125 graphics. No VT100 output.
>
> 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.
>
>
IIRC, the VT125 graphics are (surprisingly) faster than the VT240.   The
240 uses a T11; the 125 is a coprocessor 8085 (to the 100's 8080).

The 240 supported Tektronix 4010/4014 graphics in addition to ReGis. 

The VT100 was designed as a flexible platform, with lots of opportunity
(slots and power) for plugin options.  Internally, it looked more like a
computer with "bus" slots than a dedicated terminal.  (But no, the
actual uP bus of the main board  isn't exported - except to the Advanced
Video option.)  The actual implementations looked more like a loosely
coupled distributed system.

The STP (="standard terminal port" - strange white) connector is a
shorting connector.  When no card is inserted, the A and B side contacts
touch, and the serial lines pass through.  When an STP card is inserted,
the connection is mechanically broken; the STP card carries both sides
to the 8085, which creates a logical connection.  The devices negotiate
speed/flow control with control sequences.  The same connector (and
mechanism) is used by the Printer Port option.  Passive taps are also
possible.

The 125 graphics processor is completely independent of the VT100 text
processor - modulo some timing signals.

The resulting intensity signal is combined with (essentially) an analog
mixer; the color signals are separate.  This means that ANSI (VT100)
text and ReGis graphics can be independently overlaid on the screen;
something subsequent terminals (including the VT240) do not support.

The VT105 uses the same scheme, with a different board to produce
monochrome graphics.

All this flexibility came at a cost - multiple boards and connectors.
The VT101/102 are cost-reduced versions that consolidate the hardware
and eliminate the overprovisioning.

The 240 got rid of the Intel uPs for both technical and political
reasons.  The one T11 had to handle everything, including scan
interrupts.  This probably explains why it seemed slow - but I didn't
have to dig into the 240 internals to the same extent as the VT100.

The 240 and subsequent terminals never went as far in providing the
ability to add options as the VT100 did.


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