HP2648A terminal

[Bob Shannon]

First off, I meant to write DEC VT-11, not 1350.

As for digital versus analog vector generators, there is a vast difference.

And this is not like the difference between microprogrammed and random
logic designs!

A digital vector generator uses counters that drive DAC's, each counter
is clocked at a rate that controls the slope of the vector.  Most often
binary rate multipliers are used to divide a fixed clock down to the rate
needed to clock the counters up or down at the rate needed to draw
at the desired angle.

An analog vector generator on the other hand will recharge a high
quality capacitor (often compensated).  Once the X and Y axis DACS
have charged these caps to the vector starting point voltages, the DAC's
are updated with the vector end-point values an analog switching then
causes the caps to charge or discharge to the new values.

In terms of screen quality, digital vector generators often have visible
steps to the vectors, analog generators should never show such steps.

Digital vector generators 'move' the beam around by changing the
digital values to the X and Y axis DAC's over time.  Analog vector
generators (generally) only update the DAC's for the start and end
points of the vectors.

I highly recommend reading 'The secret life of vector generators", which
you can find on the web, it was written by an engineer at Atari many
years back.

Often its possible to tell an analog vector generator from a digital one
just by looking at the display quality.  Some old arcade games used
digital generators, others analog.

IMO, digital vector generators are an exercise in sheer brute force 
engineering
while the analog vector generators require a lot more finesse and attention
to details.

Clearly HP had a excellent analog design engineers, and they really worked
on getting the HP 1350 and 1351 analog sections just right.  DEC's focus
was more on the digital side, so the VT-11 offers all sorts of graphing 
modes
and other features that the 135x vector boxes would have to emulate in
software.

I think the VT-11 and HP 1350 really illustrate two very different 
approaches
to vector graphics in many ways beyond the vector generation methods.

The VT-11 takes up main memory bandwidth, while the 135x has its own
display list memory.

The VT-11 sits right on the main CPU bus and acts like a co-processor
while the 135x is very clearly a peripheral device (HPIB interface).

Sadly, the HP 135x lacks a light pen, but offers ways to logically partition
the display list for easy display animation.  This is harder (but not too 
hard)
on the VT-11 (or an Imlac for that matter).

Does the VT-11 have a keyboard port?

----- Original Message ----- 
From: "Tony Duell" <ard at p850ug1.demon.co.uk>
To: <cctalk at classiccmp.org>
Sent: Monday, August 08, 2005 6:05 PM
Subject: Re: HP2648A terminal


>>
>> I think I have to reluctantly give the nod to the DEC 1350 in terms
>
> What the heck is a DEC 1350? A PDP11 with an IB(V)11 card driving an 
> HP1350?
> :-)
>
>> of capability.  But I think the HP's analog vector generator outperforms
>> DEC's digital vector generator.
>
> In what way is the HP vector generator analogue? I've just looked in the
> service manual, it has subtractors to calculate the length, XOR gates to
> get the absoule value of that number, counters for the X and Y positions,
> rate multipliers to clock the counters based on the X and Y differences,
> etc. The output of the counters feed the DACs, the outputs of the DACs go
> to the output BNCs.
>
> It's not waht I'd call an analogue vector generator, where a couple of
> ramp voltages are generated using analogue circuitry based on the current
> and end positions of the beam.
>
> -tony
>