SP3239E

Documentation

• Datasheet

  • SP3239E Intelligent +3.0V to +5.5V RS-232 Transceiver

• Application Notes

  • ANI-10, Charge Pump Capacitor Selection Guide for 3V RS-232 Products

• Product Brochure

  • Interface Brochure

• Product FAQs

  • SP3239E FAQ

Application

Packaging

Parts & Purchasing

Quality & Environmental Data

Click on the links above to download the Certificate of Non-Use of Hazardous Substances.

Support

Search our list of FAQs for answers to common technical questions.
For material content, environmental, quality and reliability questions review the Quality tab or visit our Quality page.
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Frequently Asked Questions

What is the maximum cable length and data rate for RS-232 and RS-485 / RS-422?

For RS-232 it is 50 feet (15 meters), or the cable length equal to a capacitance of 2500 pF, at a maximum transmission rate of 19.2kbps. When we reduce the baud rate, it allows for longer cable length. For Example:

 

Baud Rate (bps)	Maximum RS-232 Cable Length (ft)
19200	50
9600	500
4800	1000
2400	3000

 

For RS-485 / RS-422 the data rate can exceed 10Mbps depending on the cable length. A cable length of 15 meters (50 feet) will do a maximum of 10Mbps. A cable length of 1200 meters (4000 feet) will do a maximum of 90kbps over 24 AWG gauge twisted pair cable (with 10 pF/ft). Refer to Annex A TIA/EIA-422-B.

 

 

Why do most RS232 devices need 4 external capacitors?

RS-232 uses both positive and negative voltages for signaling. The RS-232 driver needs a charge pump circuit to generate these signal voltages from a single Vcc supply. Four capacitors are needed to generate the positive (V+ or Vdd) and negative (V- or Vss) voltages.

Are your parts 100% tested for ESD?

ESD tests are “destructive tests.” The part is tested until it suffers damage. Therefore parts cannot be 100% tested in production, instead a sample of parts are characterized during the product qualification. The test procedure consists of “zapping” pins with a given voltage using the appropriate model and then running the part through electrical tests to check for functionality or performance degradation.

Why should I choose RS232 over RS485? What are the advantages of one protocol over another?

RS232 is the most widely implemented serial interface in the world. It is commonly installed as the serial port (9 pin or 25 pin) on PCs and has become ubiquitous on literally thousands of other applications. See below for comparisons.

 

Even though RS232 is a very old standard (first standardized in 1962) it is still popular because it is:

- simple, no software stack required, can be used to bring-up microcontrollers or load firmware on a “bare” system

- inexpensive, standard products exist from multiple vendors

- widely understood, support is already built in to most microcontrollers, the basics of serial communication are in most of the textbooks

- performance is adequate for many applications, simple data transfer, text or console ports, diagnostics, peripheral connectivity, etc.

 

However RS232 does have some limitations:

- It is slow by modern standards. Typical data rates are 1200 baud, 9600 baud, 115.2kbaud. High data rate RS232 devices are available up to 1Mbps. Faster speeds are uncommon.

- Signals swing to both positive and negative voltage. This requires an onboard charge pump to generate signals from a single power-supply chip or else multiple positive and negative supply rails.

- High pin-count per function. All signals are unidirectional and the charge pump requires several pins and external capacitors. So small footprint is difficult to achieve. Cables and connectors use more pins and wires than most modern serial protocols.

- Point-to-point only. Signals go from one driver to one receiver. RS232 does not support bi-directional signals or multiple drivers or receivers.

- Limited distance. RS232 uses single-ended signals which makes it difficult to support long cables. Typical RS232 cables are only about 10 meter or less. High speed (1Mbps) are typically less than 1 meter. The wide driver signal swing makes crosstalk a problem. Unbalanced signals with a shared ground reference are less able to withstand ground shifts between driver and receiver.

- Comparatively high power consumption. The wide signal swing takes quite a bit of power. By the RS232, signals idle at mark-state and receivers have typical 5kΩ impedance to ground, therefore drivers are constantly sourcing current even while idle. Many later RS232 transceivers’ feature shutdown modes or automatic power saving features (such as Auto On-Line, Auto On-Line Plus, Intelligent charge pumps, etc.). However some of the most commoditized devices lack any shutdown function.

 

RS485 overcomes most of the limitations of RS232 and is an excellent complement to RS232.

- RS485 uses differential signaling and is capable of much higher data rates (up to 20Mbit/sec)

- Differential signals also allow RS485 to communicate over 1200 meter cable lengths. Longer runs are possible with some careful system optimization.

- Bi-directional and multi-drop operation. RS485 can be used to build multidrop networks with many transmitters and many receivers.

- Balanced differential signalling also makes RS485 highly immune to noise. On twisted-pair cables a noise signal will couple equally to both wires in the pair and be ignored by the differential receiver.

 

RS485 is found mainly in industrial, telecom and commercial applications and is not as widespread in the consumer

or PC world. Therefore it is not seen as often as RS232.

 

Also the RS485 protocol standard defines only the electrical characteristics of the interface. The physical and logical implementations are left up to the user. Different connectors, different methods for bus-arbitration and data framing all exist under a wide variety of implementations. RS485 has also been used as the foundation for many proprietary or semi-proprietary standards. Therefore interoperability between RS485 based interfaces is not always as simple as with RS232.

How much ESD voltage could a part see in normal use?

ESD is caused by static electricity. In order for an ESD event to occur there must be a buildup of static charge. Very high charge levels are actually quite rare. In a normal factory environment, taking basic ESD precautions (grounding-straps, anti-static smocks, ionizers, humidity control, etc.) static levels can be kept below a few tens of volts. In an uncontrolled environment, like an office, static levels rarely get above 2000 volts. Under some worstcase conditions (wearing synthetic fabrics, rubbing against synthetic upholstered furniture, extremely low humidity)

levels can go as high as 12 to 15 thousand volts. Actually to get to 15000 volts or higher you would need to be in an uncomfortably dry environment (humidity below 10%) otherwise static charge will naturally dissipate through corona discharge. It would definitely be considered a “bad hair day.” Humans can generally feel a static shock only above 3000 volts. A discharge greater than 4000 volts can cause an audible “pop.” But repeated lower level discharges can be imperceptible and still may have a cumulative damaging effect on sensitive ICs. All ICs, even those with robust protection, can be damaged if they are hit hard enough or often enough.

Why are some pins protected to higher ESD levels than others?

Most ICs in a typical system are at greatest risk of ESD damage in the factory when the PCB is assembled and the system is being built. After the system is put together they are soldered onto the PCB and shielded within a metal or plastic system enclosure. Interface ICs are designed to attach to an external connector that could be exposed to ESD when a cable is plugged in or when a person or object touches the connector. These interface pins are most likely to see ESD exposure and therefore benefit from additional protection.

Some RS-485 transceiver part numbers look similar to each other but they have an “E” in the part number. Others do not. What does the E mean?

Actually the letter “E” could have two different meanings, depending on where it is in the part number. Most of our interface devices are available in different temperature grades. Commercial temperature (0 to 70C) has a “C” after the numeric part number. Industrial-extended temperature (-40 to +85C) use the letter E. So for example SP485CN is commercial and SP485EN is industrial. The second letter indicates the package type, in this case N for narrow-SOIC. Another E in the suffix indicates that this device has enhanced ESD protection, typically of ±15000Volts on the interface pins. Devices that do not have the enhanced ESD still contain built-in ESD protection of at least ±2000Volts. For example the SP485ECN is ESD rated up to ±15kV, and the SP485CN is rated for ±2kV HBM.

How can I find out if the status of a part number is EOL / obsolete or active?

The best way to determine this is to go to maxlinear.com and type the part into the search function. At or near the top of the results you should see something that looks like:

 

 

 

In this example, we looked for XRA1201. Click on part number header or on the link at the bottom of the result.  This brings you to the product page. For example:

 

 

 

Click on Parts & Purchasing, highlighted with the blue arrow above. The screen changes to:

 

 

 

Notice the status column and the “Show obsolete parts” link.  A legend tells you the definition of the different statuses. Click on the “Show obsolete parts” link to see EOL or OBS part numbers along with the Active part numbers, the legend still present:

 

 

 

Another method to find out if a part is OBS or EOL is to click on SUPPORT at the top:

 

 

 

And then Product Change Notifications:

 

  

 

Type the part into the search, and click on one of the part numbers from the drop down menu. Then you can look for the Product Discontinuation Notice, which generally is at the top of the list, for example:

 

 

 

If you see this, it tells you that this particular orderable part has been discontinued and when the last order date is, or was. If you click on the file, then you can view the notice we sent about this if you purchased the part in the recent past. It may also advise of a replacement part. When an orderable part first becomes discontinued, Product Discontinuation Notices are sent are sent to those who have purchased the parts in the recent past, if purchased directly, with a dated opportunity to place a last order.

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