INTRODUCTION
by Richard Parkinson
A key component in any data communications network is the interface that connects data terminal equipment (DTE) to data circuit-terminating equipment (DCE). Although the Electronic Industries Association (EIA) RS-232C inter-face standard has served the industry well and is still widely used, the demands.
of new applications have exceeded its capacity. Fortunately, a relatively new standard, EIA RS-449, is gaining momentum and seems destined to replace RS-232C. This chapter discusses the evolution and capabilities of this new interface, its compatibilities with other standards, and its future.
TheEIA
EIA is an association of electronic equipment manufacturers in the United States; the association is also a member of the American National Standards Institute (ANSI). An EIA subcommittee is responsible for interface compatibil-ity, enhancing existing standards, and/or developing new standards. Working papers are developed by committee members, with several iterations that include input from outside interested parties; a final draft is voted on by the EIA general committee. This was the procedure followed in the development of RS-449 and its companion standards, RS-422 and RS-423.
Defining an Interface
The interface discussed here connects a DTE (a terminal device or com-puter) to a DCE (a traditional to-analog modem or line driver [a digital-to-digital device)). Figure 5-1 shows these two components joined by a multi-wire cable. Each multi-wire lead passes low-voltage electrical signals between the two devices, with the direction of flow (to or from the DCE) specified by the EIA standard. Each wire is assigned one of four basic functions:
• Data transfer
• Control signals
• Timing
• Signal grounds
Interface Connector
Multiwire Interface Cable
Figure 5·1. Joining a DYE and a DeE DCE
Both RS-449 and its predecessor, RS-232C, perfonn these basic functions over wires that conveniently tenninate in a connector that joins the DTE and DCE.
Thus, the three components of an interface are functional information transmit-ted by electrical signals over wires that terminate in a mechanical connector.
EVOLUTION OF THE INTERFACE
RS-232C was (and still is) the most predominant DTEIDCE interface standard since the early 1960s. Improved technology and application demands, however, have exceeded the interface's capabilities (a fact that was recognized in the early 1970s). In today's environment, RS-232C carries such deficiencies as:
• A maximum DTE-to-DCE signaling rate of 20,000 bits per second.
• A maximum distance between DTE and DCE of, nominally, 50 cable feet; in practice, with shielded cable, a distance of several hundred feet is possible.
• Some manufacturers arbitrarily assign features not supported by RS-232C to any unused pin.
• Because RS-232C did not specify any particular mechanical interface connector, many different and incompatible designs proliferated; some connectors, for example, were attached to the DTE with screws and others with clips.
Because the first two points on this list were the most troublesome, these areas were addressed first. The results were the RS-422 standard for balanced circuits and the RS-423 for unbalanced circuits, both of which were issued in 1975. Although detailed electrical engineering information on the differences is beyond the scope of this chapter, it is available in the standards documents (see Suggested Readings). The following information, however, should be sufficient to understand why RS-422 and RS-423 provide better perfonnance characteristics than RS-232C. Figure 5-2 shows the three approaches to electri-cal signaling between a DTE and a DCE. With RS-232C, both the generator and receiver circuitry share a common ground wire in both directions (pin 7, signal ground). Sharing a common ground reduces the immunity to the noise caused by one circuit's signals interacting with the signals of another. (This phenomenon manifests itself in an automobile tape deck or radio as a
high-r - - - -,
DTEI
I
I
- I
L ________
-.J
RS-232C
RS-423
RS-422
r---,
DCEI I
I I _ L _____ -=--__
Figure 5-2. Balanced versus Unbalanced Interchange Circuits
pitched whine that varies with engine revolutions per minute. In this example, the radio ground is not isolated from other signal grounds and causes the interference.) This common ground, more than any other, is the factor that limits speed and distance when using RS-232C.
RS-423 provides for unbalanced electrical characteristics where the genera-tor at either end shares a common ground, but the RS-423 receivers have separate grounds. The primary application of RS-423 is to provide compatibil-ity with RS-232C when a DTE or DCE has the opposite interface to its
connected partner. This allows transition from RS-232C equipment to RS-449 as needed.
RS-422 provides balanced electrical characteristics where both generator and receiver use independent ground wires. This isolation of grounds allows greater capability than does RS-232C or RS-423.
Although the RS-232C standard covers the functional, electrical, and me-chanical components, RS-422 and RS-423 address only the electrical require-ments. A new standard was therefore needed to cover the functional and mechanical requirements. That standard, issued by EIA in 1977, is RS-449.
The remainder of this chapter deals primarily with the functional aspects of this relatively new standard that are of primary interest to data communications users.
THE STANDARD
Of the advantages offered by the RS-449 standard over its predecessor, RS-232C, the most notable are:
• Signaling rates over the DTE/DCE interface of up to 2M bits per second (see Table 5-1), which is considered a conservative practical speed. For shorter distances, the full RS-422 signaling limits would be applicable.
• A distance between DTE and DCE of up to 4,000 feet at approximately lOOK bits per second (see Table 5-1).
• Addition of 10 new circuit functions.
• Elimination of three RS-232C circuits.
• Segregation of secondary-channel circuits into a separate 9-pin connec-tor.
• Use by the primary circuits of a new 37-pin connector (the mechanical characteristics of these two connectors form part of the standard).
It is essential that users understand the capabilities of RS-449, both to ensure its efficient utilization and to diagnose problems attributable to either the DTE or the DCE. Appendixes A and B contain equivalency tables that show the functional assignments of the pins for both RS-232C and RS-449. For clarity, the comparison is based on the four duties of these leads (i.e., grounds, data, control, and timing). In the primary channel equivalency table (see Appendix A), it should be noted that some functions show two pins assigned while others show only one. Functions with only one pin are referred to as Category IT
Table 5-1. Comparison of Distance versus Data Signaling Rate Distance (ft) RS-232C RS-423 RS-422
10 ~20K bps >100K bps 10M bps 40 ~20K bps 100K bps 10M bps 50 20K bps § 90K bps §i 9M bps 100 <20K bps § 80K bps § SM bps
1,000 NA § 6K bps § SOOK bps
4,000 NA §9oo bps § 100K bps
circuits; those with two pin assignments are called Category I circuits. The latter are used for data signaling and control functions that must respond very rapidly at high data rates. Category IT circuits are those that are not required to change their state quickly, so sharing a common ground is quite adequate.
Appendix C compares pin assignments of RS-449 and RS-232C and pro-vides a brief functional description of each RS-449 interface lead. (There is a great deal of similarity with RS-232C functions; some have new names but perfonn the same job.) One improvement provided by RS-449 is the designa-tion of mnemonics that more closely relate to the actual funcdesigna-tions. "Clear to send," for example, was given the mnemonic CB with RS-232C; with RS-449 it is CS. In practice, however, the more common mnemonic for clear to send has been CTS, and it is doubtful that the advent of RS-449 will quickly persuade users to move from CTS to CS.
RS-449 also provides more substantive features in addition to improved speed and distance. Most of these new functions are service oriented, allowing testing and restoral capabilities under DTE control, as shown in Figure 5-3.
Of particular interest are the select standby (SS) and standby indicator (SB) leads (shown in Figure 5-3a). By turning on the SS lead (pin 32), the DTE instructs the modem to use an alternate standby facility. This would, for
a Select Standby and Standby Indicator Function 4-wire
00
circuit R5-449