(i) Graphic LCD

JHD12864J is a light weight, low power consumption liquid crystal graphic display. The module measures 54.0x50.0mm only. Supply voltage is 5V matching the voltage for most microcontrollers.

Unlike most character-based LCDs which use 4-bit data bus, JHD12864J module uses 8-bit data bus (DB0 – DB7). JHD12864J is split logically in half with controller #1 (CS1) driving the left half of the display, and controller #2 (CS2) driving the right half. These two portions map directly to the physical display area as shown in Figure. With a correct controlling sequence on pin CSx (x=1,2), D/I, and R/W, we can write any pattern say, 0xAB directly to the LCD screen at a designated column position.

It is illustrated in Figure below:

Display Control Instruction

There are two ways to interface the LCD with x51:

1. Address/data bus memory map, i.e. use the LCD as a piece of external RAM
2. Direct I/O connection by writing high/low signal to LCD.

(ii) RS 232/485

In the early 1960s, a standards committee, today known as the Electronic Industries Association (EIA), developed a common interface standard for data communications equipment.

At that time, data communications was thought to mean digital data exchange between a centrally located mainframe computer and a remote computer terminal, or possibly between two terminals without a computer involved.

These devices were linked by telephone voice lines, and consequently required a modem at each end for signal translation.

While simple in concept, the many opportunities for data error that occur when transmitting data through an analog channel require a relatively complex design.

It was thought that a standard was needed first to ensure reliable communication and second to enable the interconnection of equipment produced by different manufacturers, thereby fostering the benefits of mass production and competition. From these ideas, the RS232 standard was born. It specified signal voltages, signal timing, signal function, a protocol for information exchange and mechanical connectors.

RS 232 C uses voltage level +3 V to +15 V for logic 0 and – 3 V to – 15 V for logic 1. As these voltage levels are not compatible with TTL logic levels, the line drivers and receivers are used to interface RS 232 and TTL devices. The RS 232 C specifies a maximum distance of 50 feet at a maximum baud rate of 20 K. For lower baud rates the distance may increase from 2000 to 3000 feet.

Fig. below shows the interface between TTL and RS 232 signals.