The Securities and Exchange Commission yesterday set a Sept. 30 deadline for the securities industry to come up with a plan for linking the various stock markets and a system for routing stock orders between those various markets.

This will be a major step toward formation of a national market system, which was ordered by Congress in 1975. The commission began work on the national market system almost seven years age.

In releasing the 58-page blueprint for the system. Chairman Harold M. Williams suggested that the SEC proposals was a way to test whether the industry was going to continue to drag its feet.

"The question is, whether the industry is going to continue to think of itself in pieces," Williams said. And he added: "This is a good way to find out?"

The exchange most seriously affected by the national market plan is the New York Stock Exchange, whose rules limiting access to listed stocks was a major reason for the congressional action.

Spokesmen for the New York Stock Exchange and for the other major exchange, the American Stock Exchange, said officials were reserving comment until they could study the SEC document.

In separate but related action, the SEC ordered all exchanges and so-called third markets to make available quotations on all traded stocks.

Currently, there are no rates requiring exchanges to disseminate bids, offers and quotation sizes to all of the investing public. In explaining the new rule, which becomes effective May 1, the SEC said: "The rule is intended to facilitate the prompt development of a composite quotation system, an integral component of a national market system."

The idea of the national market system is to allow investors anywhere in the country to shop fo r the best possible price on a stock that is traded on any number of exchanges or in the third market."

For example, an investor seeking to buy stock in XYZ Corp, will be able to get quotations from the Bosten Exchange, the Pacific Exchange and the NYSE, where the stock might be trading. It may be that XYZ is selling and &8th of a point less in Boston, and the buyer, who may be in Denver, can put an instant buy order to the Boston exchange.

One major obstacle to this system is the NYSE's so-called Rule 390, which states that certain types of transtations must be done only through the New York exchange.

In a press briefing, Chairman Williams indicated that Rule 390 would eventually have to go if a national market system is ever going to be established. But he refused to say that it [PARAGRAPH ILLEGIBLE]