subprocesses check the flags and issue an error return if the operation cannot be successfully completed. Note that the complete signal is given when the error signal is set (why?). Can this spurious complete signal be eliminated?

2. The K(arbiter) presented above used a polling loop to check the request flags. In some applications, however, such a loop might not be fast enough to allow efficient use of the shared facility. Design a combinational circuit to replace the polling loop in the K(arbiter). The rest of the structure can remain unchanged.

TRANSDUCERS FOR DATA COMMUNICATIONS SYSTEMS

KEYWORDS: Synchronous, asynchronous, communications, timing, receive, transmit.

This section will examine a class of transducers which are used to transfer data between physically separate sites via communications links, usually conventional telephone lines. They are presented because nearly all digital systems engineers eventually are involved with digital data communications and this section will serve as a brief introduction. It is also hoped that when users understand how really simple data communications are, these concepts, formats, and interfaces will be used instead of designing new interfaces which are incompatible with everything else.

James Martin has written a series of books (e.g., 1969) on digital communications. Knowledge in this area is based on classical communications theory together with a large variety of equipment. The digital communications professional should know: classical communications theory, various telephone company line tariffs and policy, equipment that is available and which can be legally interfaced to lines provided by the telephone communications companies, what types of equipment can use the facilities, etc.

An overview of the system under study is given in Figure TRAN-1. The telephone companies (i.e., communications carriers) fundamentally provide communications lines (links) between pairs and larger groups of subscribers, allowing them to communicate audio, picture phone, video, and digital data information. They also provide facsimile devices and Teletypes for the transmission of printed information. Finally, they allow subscribers to select connections to other subscribers (i.e., switching). To be considered along with the telephone companies are federal, state and local regulating commissions, etc. This large, internal mass, Figure TRAN-1, is considered as given -- with no clear input or output.

As one moves away from the central hard-core of communications suppliers that fundamentally provide a switched, twisted pair of wires between one site and another, the next system obstacle is encountered. Since the twisted pair of wires usually transmits data in the audio frequency range of 100-3000 Hz, there has to be some transducer to convert information in that frequency domain into the 0 and 1 needed by the digital engineer. The transducer which converts this audio frequency data into digital data bits (0 and 1) is called a modem. The conversion encoding is implemented by using one of the following': amplitude modulation (AM) with on-off keying; frequency modulation (FM), using frequency shift keying; and phase modulation (PM). For the vast number of people who profess to be communications engineers and who do not work for the telephone company or the government (which regulates the telephone companies) the modem is simply another given component. There are exceptions, since a ruling of one of the regulatory agencies does allow a user to provide his own modem in certain cases; user provided modems are common when low cost is a design

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