Many applciations arise when you might want to use either a cut down BBC (just the processor, some RAM and an IO port) in, for example, a burgular alarm system or when it is not practical to use a 14" monitor (eg a shop cash register).

Use of a liquid display crystal display would alleviate the need for complex colour video generating circuitry.

The system described in this article uses the Hitachi LM044L 20 column by 4 line LCD module available from electronics distributors.

Parts

The above circuit diagram shows electrically how to assemble the unit. It requires the following parts:

1	x	LM044L (Farnell Part Number: 325429)
1	x	10k variable resistor
1	x	5v power supply (eg 4 x AA batteries)
1	x	20 way IDC user port connector
1	x	Length of 20 way ribbon cable

Benefits

• No bulky/unreliable cathode ray tubes



• Far more compact



• Runs off 5v supply



• Could be used with cut down BBCs in embedded applications



• Screen contrast is adjustable in poor viewing situations

How To Build

1. Simply split the 20 way ribbon cable and connect as suggested in the diagram,



2. add the 10k potential divider to control the contrast across the marked pins on the diagram. Be sure to get the gold card edge connectors the right way round,



3. add the power supply, and



4. crimp the IDC plug on the other end of the ribbon cable and connect to the BBC micro's user port.

Hints

• Since LCDs are so efficient, the 5v supply can be derived from the two 5v pins on the BBC's user port plug.



• Run the software with *LCD. It loads below the Mode 7 screen memory.



• The software implements all of the VDU commands that are supported by OS1.20 and that are related to text operations. For example, you can't do any graphics plotting!



• The cursor is controlled by the use of VDU23 allowing it to be turned off in applications that require this.



• The whole VDU queue is then passed on the operating system so output will appear on both the LCD and the normal BBC's screen.

Happy soldering from Robert Sprowson!