Part 1 looked at the basics of RS-232. This part continues that examination, along with some extensions to the standard and test issues.
Q: What are some of the issues associated with RS-232 and its performance?
A: The standard allows for many user choices in how the DTE and DCE are configured: number of data bits (from 5 to 8); parity bits (odd, even, or none), number of stop bits (1, 1 ½, 2), and speed (nine settings from 110 through 19200 baud). In addition, as technical demands increased and voltages decreased, many somewhat-compatible but non-standard (proprietary) variations emerged, using lower voltages, higher speeds, and smaller, unique connectors.
As a result, RS-232 interconnections are often frustrating to establish in a given situation. Further, once it is set up and working, users are reluctant to change it or substitute a new unit. That has extended the legacy situation.
Q: What role do IC vendors have with respect to RS-232?
A: An RS-232 interface does not require ICs or even active devices. However, as the use of RS-232 grew – and it was a very widely used interface, despite the many challenges – IC vendors responded with line drivers and line receivers which simplified the interface design issues, including providing translating of the low, unipolar voltages within a circuit (usually at 12 V, 5 V, or 3 V) into the bipolar voltages needed. Some of the ICs also to take an “unframed” data byte and do all the needed formatting and timing of start, stop, and parity bits. These IC often provide some protection against line transients, which are common on long lines with high-impedance voltage signals.
Q: What other tools are used with RS-232 interlaces?
A: A typical RS-232 interface uses hard-wired settings for operating parameters, or allows users to set them via jumper wires or small switches. Nevertheless, there are many cases were the DTE and DCE would not “talk”, often due to issues to the control lines. For this reason, low-cost “breakout boxes” were often used which brought all the connector lines to test points, so they could be monitored on an oscilloscope or other instrument, Figure 1.
These breakout boxes often have small DIP switches so RS-232 lines could by “cut off” and then “jumpered” to other lines, to see if a different configuration would work. The breakout box is a static, passive unit; for more-advanced troubleshooting and debugging, an instrument called a “line monitor” is needed to observe and dynamic display line status, data lines and bit patterns, and other conditions.
Q: What is a “null modem?”
A: Null modem is a term associated with RS-232 interconnections. It is needed because the RS-232 standard presumes that one device is configured as a DCE and the other as a DTE. However, in many situations, two DCEs (or DTEs) are trying to connect to each other, since there is no formal standard for defining which devices are DTEs and which are DCEs; it is mostly determined by “what seems to make sense” and so different people and installations have different views.
To get around the problem, a specific cable called a null modem is used, Figure 2, which re-arranges the original straight-through cable (pin 1 to pin 1, and so on) so that the DTE is made to look like a DCE, or vice versa. Different null-modem cables are available as standard items to meet the various installation challenges. In most null-modem versions, pin 2 would be wired to pin 3 as a starting point, to transform a DTE into DCE.
Q: We’ve looked at RS-232 in detail, but what about RS-422, RS-423, and RS-485?
A: Despite its limits in distance, speed, and performance, RS-232 was the only viable interconnection for many years, with tens of millions of RS-232 ports in use. However, even when it is working properly, it is a point-to-point interconnect, with one source and one receiver connected directly to each other.
To overcome these limitations and expand the use of the RS-232-type of interface, some additional versions were defined and subsequently supported by ICs.
Q: What are RS-423, RS-422, and RS-485?
These are three most-common enhancements to the basic RS-232 standard:
–RS-423 increases the data rate to 100 kbps and the distance to 4000 feet, and allows for up to ten line receivers all sourced from one common driver. This allows for applications such as a single computer driving ten identical displays or readouts in parallel. The increase in speed is achieved by lowering the maximum voltage to -3.6 to -6.0 V for a binary one and +3.6 to +6.0 V for a binary zero; this reduces the required slew rate by a factor of 4 compared to RS-232. Like RS-232, the signals are all single-ended.
–RS-422 increases the data rate to 10 Mbps, at distances up to 4000 feet, while allowing for up to 10 receivers. Signal levels are similar to RS-423, with -2.0 to -6.0 for a binary 1 and +2.0 to +6.0 V for a binary zero. The major change is that the signal lines are no longer single-ended, with reference to a common ground. Instead, they are differential, with a pair of wires for each signal; this eliminates the problems of noise and shift which occur with a single ground reference, but adds to the number of psychical wires needed.
–RS-485 is an enhanced version of RS-422 with the same speed and distance, and supports multidrop operation. This means that more than one driver can be on the link, although only one can be active at any time. The drivers not in use are directed to be disabled and to go to a high-impedance state, so it appears they are not on the line at all. All receivers can be active at the same time, as there is no signal conflict due to multiple receivers just “listening;” the circuitry associated with these receivers can choose to ignore any signals. R-485 allows for up to 32 receivers, with -1.5 to -6.0 V for a binary 1 and +1.5 to +6.0 V for a binary 0.
Despite its age and relatively low performance, there are cases where RS-232 is still an acceptable connectivity solution, or one which must be understood for legacy and retrofit considerations.