HC 77B Scrolling LED Display

HC-77B Scrolling LED Display

Release Identity

Field	Detail
Product	HC Scrolling LED Display
Release wave	Mid 1978
Display	32×8 monochrome red dot matrix
Pixel count	256 LEDs
Construction	Four close-fitted 8×8 LED modules on an exposed PCB/backplane
Power	Separate local supply and on/off switch
Host interface	Memory-mapped I/O extension

The LED Display is a directly programmed peripheral. It has a 32-byte visible display store and an HC character-ROM lookup facility. It does not contain a text screen, autonomous scroll engine, or high-level graphics processor.

Display Model

The 32 display bytes represent vertical columns from left to right:

$F750 = leftmost display column
$F76F = rightmost display column

Within each column byte:

bit 7 = top LED
bit 0 = bottom LED

A program can read or write every column immediately. The sign refreshes its LED matrix internally at a rate high enough to appear steady; scan timing is not part of the programming interface.

I/O Map

Address	Function	Access
$F750-$F76F	Display columns 0-31, left to right	Read/write
$F770	Font-select latch	Read/write
$F771-$F778	Font columns 0-7, left to right	Read only

Writes to $F771-$F778 are ignored.

Power Behaviour

The extension is fitted through the normal emulator extension system but represents a separately powered physical board.

• On first attachment, local power is off.
• When off, reads from $F750-$F778 return $FF; writes are ignored.
• A local off/on cycle clears all 32 display bytes and the font-select latch to $00.
• HC-77B warm reset does not clear the sign.
• Closing the sign window only hides the view; the device remains attached and active.

Font ROM

Write an HC character code to $F770, then read its eight vertical glyph columns from $F771-$F778.

The sign uses the same character codes and artwork as the HC Keyboard/CRT display, stored in a column-oriented form suitable for the vertical LED columns. Unsupported character codes select the blank/NUL glyph.

A conventional right-to-left text scroll:

1. Copy display columns $F751-$F76F left into $F750-$F76E.
2. Read the next font byte from $F771-$F778.
3. Write that byte to $F76F.
4. Delay in software.

The HC-77B program controls all movement, timing, spacing, and animation.

Demo Cassette

The supplied demo cassette is a base-1 KB machine-code example.

Address	Meaning
$0200	Run current settings and message.
$0380	Restore HOMECOMP - WELCOME defaults, then run.
$0050	Scroll-delay outer-loop count. Larger values are slower.
$0051	Blank columns between message repetitions; default $18 = 24.
$0052	Message length. $00 clears the sign and returns to the monitor.
$03C0-$03FF	64-byte user message buffer.

The demo clears the sign at $0200, then scrolls exactly the number of message bytes named by $0052. It does not use a terminator or bounds check. Press RST to leave the running program through the normal HC-77B monitor reset path.

Expansion-Bus Position

$F750-$F778 is a released HC-77B device specification. Third-party extensions must not reuse this range.

See HC-77B Expansion Bus API for extension packaging and address-claim rules.