The present invention relates to computers and, more particularly, to an integrated circuit (IC) which interfaces with a microprocessor for providing support function therefor.
Conventional computer systems are usually comprised of a main frame computer that interfaces with a peripherally connected keyboard via a interfacing bus similar in appearance to the coiled cord connecting a telephone headset to a telephone.
The keyboard is an alpha-numeric "typewriter" style ASCII (American Standard Code for Information Interchange) keyboard for text editing and high-level language programming. The alpha-numeric entry from the typewriter keyboard is by nature serial, or one letter at a time, so nothing is lost by presenting the information from the approximately 128 keys, to the computer in a cost effective and asynchronous serial format. Electronics located at the keyboard not only debounce the pressed key but also codes its identity and transmits the information to the main frame computer via the connecting cable bus. Usually the keyboard electronics is Transistor Transistor Logic (TTL) compatible, having a high logic level of +2.4 volts and a low logic level of +0.4 volts, for providing an approximate 2 volt noise margin. However, when data is interfaced to other remote electronics, such as in the keyboard to main frame computer cable interface application, a higher noise margin is desirable and the data signals are generally converted to larger voltage swings.
For short distance applications, and where RFI (radio frequency interference) is a consideration, the RS423 standard is a good choice because the driver can be slew rate controlled and, by sloping the rise and fall times of the square wave type data, many of the high frequency components that can contribute to RFI are avoided. The RS423 standard provides a minimum of 7.2 volts of noise margin with a single ended driver output swing of +/-3.6 volts.
Most, if not all, prior art keyboard electronics as discussed above has been constructed of standard logic blocks using many discrete components and IC's. Improved performance and a more cost effective system may be achieved using a single integrated circuit for providing certain functions.
In order to reduce component cost, on board space and manufacturing costs a single-chip microprocessor may be utilized to provide the basic digital functions of the keyboard to main frame computer interface. However, there are some digital and linear functions that are required which either the microprocessor cannot do or cannot achieve efficiently.
Hence, a need exist for a microprocessor support IC to complement the standard single IC microprocessor and to provide functions in combination with the latter to provide a cost effective and improved keyboard to computer interface system.