Shift Registers as Delay Lines (was Delay lines in TV sets)

Rod Smallwood RodSmallwood at
Fri Dec 21 02:09:21 CST 2007

The next VDU design after that was a sort of state mahine. 


-----Original Message-----
From: cctech-bounces at
[mailto:cctech-bounces at] On Behalf Of Tony Duell
Sent: 20 December 2007 21:58
To: cctalk at
Subject: Re: Shift Registers as Delay Lines (was Delay lines in TV sets)

> I actually worked on VDU's that used shift registers as the display 
> memory around 1973.
> If I remember correctly one lines worth (80 characters) got 
> transferred to a buffer in the interline gap (Fly back time). The 
> buffer was scanned out in such a way as to address the character 
> generator one row at a time. Characters were 7x5 dots so the top five 
> dots of the first character was serialized followed by the top row of 
> the second one and so on. Then all of the second character row dots on

> a line would be displayed until the end of the store (and therefore 
> frame was reached)

The HP9866A thermal printer used MOS shift registers for character
storage. I am tryign to recall the details, because it's not as obvious
as you might think. From memory : 

The printhead is the full width of the paper, with 400 eleemnts (80
charas * 5 dots/char). Physically it's made up of 4 modules. IIRC each
printhead module is divided up into 4 sectiosn, each of 5 characters,
and I think they're interleaved (so characters 0, 4, 8,... are one
section, 1, 5, 9,... are the next and so on. I might have got that
wrong, though)

Incoming characters (7 bit ASCII) are re-encoded to 6 bits (it's an
upper-case only printer, so 6 bits is enoguh) and stored in 6 80-bit
Shift registers (physcially 3 TO99 cans). 

When the printer dtects a LF character, the more comlicated parts starts
up. This reads ou the shift retgisters, every fourth character is sent
to the character geenrator ROM, and the bit pattern for the top row is
shfited into a set of 5 (one for each column of the chracters) 20 bit
shift registers (I think the chips used are 8203s) which drive the

That pattern or dots is printed

The shift registers are run rount again, now the next set of chars taken
throgh the character gernearotr ROM. to the printhead shift registers,
and printed

After the top row of all 80 characters has been printed, the paper is
moved up one dot-line, and the process is repeated, only this time the
character generatoter ROM outputs the patterns for the second row of the

And so on until the complete characters have been printed.

This is controlled by a state machine that looks simple as a schematic
(a handful of chips), but is not easy to understnad!


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