Intel 210200-002 manual ~Reset

APPLICATION EXAMPLES

that eliminate the need for the +12 volt and - 12 volt power supplies normally required. The +12 volt power supply was eliminated by connecting the emitter of Tl to +5V. While this produces a signal that is not strictly within the RS232C specification, it works well on interconnections of less than 10 meters.

This design also employs a useful trick to eliminate a -12V power supply. Many people have attempted to eliminate this supply by driving the TRANSMIT-DATA line bet- ween GROUND and +5V. Because of a circuit switching element (Transistor T2), the low-level signal is always a little higher than ground and hence won't work with many terminals requiring a negative voltage for a LOW. This design, however, uses the REC- EIVE-DATA line (presumably driven by a true RS232C-compatible terminal) as a source of a negative voltage.

This negative voltage (negative whenever RECEIVE-DATA is low) charges capacitor Cl through diode Dl. This circuit has been verified to work when receiving any sequence of characters, except BREAK.

BREAK causes a very long "I" on REC- EIVE-DATA; TRANSMIT-DATA event- ually exhausts the negative charge on cap-

acitor Cl. If desired, a -12 volt supply may be connected to the junction of Cl, Dl and R4.

This RS232C interface is driven by soft- ware to provide the proper timing for transmitting and receiving characters.

Multiplexed System #2: The Vest Pocket Computer

Combining state-of-the-art microprocessor components results in a usable computer small enough to be carried in a vest pocket (Fig. 4.2).

In only 15 square inches (3"x5"), this system could contain a 2K tiny BASIC operating system, 16K memory for user programs, and an I/O port. The port is designed to interface to a terminal.

The system is designed with an 8088 CPU, 8755A I/O Port with EPROM and 21821 RAMs with 4K byte density each.

The 21821 is a new concept in RAM architecture, interfacing directly on the iAPX 86, 88 or MCS-85 multiplexed bus, respond- ing directly to controls from the processor.

Contained within the 21821 is a com- plete memory system on a single piece of silicon.

[J			DIP----	MDS	L	D!
					s
			SWITCHDVDT		D!
rTl4		8755A-2	2	2	2	2
2
:
WA	8088		1	1	1	1
			8	8	8	8
			2	2	2	2
~RESET			1	1	1	1

Figure 4-2. Vest Pocket Computer Component Layout

4-5