The EIA/TIA-232-E standard was introduced in 1962 and has since been updated four times to meet the evolving needs of serial communication applications. The letter "E" in the standard's name indicates that this is the fifth revision of the standard.
RS-232 Specifications
RS-232 is a complete standard. This means that the standard sets out to ensure compatibility between the host and peripheral systems by specifying:- Common voltage and signal levels
- Common pin-wiring configurations
- A minimal amount of control information between the host and peripheral systems.
Electrical Characteristics
The electrical characteristics section of the RS-232 standard specifies voltage levels, rate of change for signal levels, and line impedance.As the original RS-232 standard was defined in 1962 and before the days of TTL logic, it is no surprise that the standard does not use 5V and ground logic levels. Instead, a high level for the driver output is defined as between +5V to +15V, and a low level for the driver output is defined as between -5V and -15V. The receiver logic levels were defined to provide a 2V noise margin. As such, a high level for the receiver is defined as between +3V to +15V, and a low level is between -3V to -15V. Figure 1 illustrates the logic levels defined by the RS-232 standard. It is necessary to note that, for RS-232 communication, a low level (-3V to -15V) is defined as a logic 1 and is historically referred to as "marking." Similarly, a high level (+3V to +15V) is defined as a logic 0 and is referred to as "spacing."
Figure 1. RS-232 logic-level specifications.
The RS-232 standard also limits the maximum slew rate at the driver output. This limitation was included to help reduce the likelihood of crosstalk between adjacent signals. The slower the rise and fall time, the less chance of crosstalk. With this in mind, the maximum slew rate allowed is 30V/ms. Additionally, standard defines a maximum data rate of 20kbps , again to reduce the chance of crosstalk.
The impedance of the interface between the driver and receiver has also been defined. The load seen by the driver is specified at 3kΩ to 7kΩ. In the original RS-232 standard the cable length between the driver and receiver was specified to be 15 meters maximum. Revision "D" (EIA/TIA-232-D) changed this part of the standard . Instead of specifying the maximum length of cable, the standard specified a maximum capacitive load of 2500pF, clearly a more adequate specification. The maximum cable length is determined by the capacitance per unit length of the cable, which is provided in the cable specifications.
Table 1 summarizes the electrical specifications in the current standard.
Table 1. RS-232 Specifications
| RS-232 | |
| Cabling | Single-ended |
| Number of Devices | 1 transmit, 1 receive |
| Communication Mode | Full duplex |
| Distance (max) | 50 feet at 19.2kbps |
| Data Rate (max) | 1Mbps |
| Signaling | Unbalanced |
| Mark (data 1) | -5V (min) -15V (max) |
| Space (data 0) | 5V (min) 15V (max) |
| Input Level (min) | ±3V |
| Output Current | 500mA (Note that the driver ICs normally used in PCs are limited to 10mA) |
| Impedance | 5kΩ (Internal) |
| Bus Architecture | Point-to-Point |
Functional Characteristics
Because RS-232 is a complete standard, it includes more than just specifications on electrical characteristics. The standard also addresses the functional characteristics of the interface, #2 on our list above. This essentially means that RS-232 defines the function of the different signals used in the interface. These signals are divided into four different categories: common, data, control, and timing. See Table 2. The standard provides abundant control signals and supports a primary and secondary communications channel. Fortunately few applications, if any, require all these defined signals. For example, only eight signals are used for a typical modem. Examples of how the RS-232 standard is used in real-world applications are discussed later. The complete list of defined signals is included here as a reference. Reviewing the functionality of all these signals is, however, beyond the scope of this paper.Table 2. RS-232 Defined Signals
| Circuit Mnemonic | Circuit Name* | Circuit Direction | Circuit Type |
| AB | Signal Common | — | Common |
| BA BB | Transmitted Data (TD) Received Data (RD) | To DCE From DCE | Data |
| CA CB CC CD CE CF CG CH CI CJ RL LL TM | Request to Send (RTS) Clear to Send (CTS) DCE Ready (DSR) DTE Ready (DTR) Ring Indicator (RI) Received Line Signal Detector** (DCD) Signal Quality Detector Data Signal Rate Detector from DTE Data Signal Rate Detector from DCE Ready for Receiving Remote Loopback Local Loopback Test Mode | To DCE From DCE From DCE To DCE From DCE From DCE From DCE To DCE From DCE To DCE To DCE To DCE From DCE | Control |
| DA | ransmitter Signal Element Timing from DTE | To DCE | |
| DB DD | Transmitter Signal Element Timing from DCE Receiver Signal Element Timing from DCE | From DCE From DCE | Timing |
| SBA SBB | Secondary Transmitted Data Secondary Received Data | To DCE From DCE | Data |
| SCA SCB SCF | Secondary Request to Send Secondary Clear to Send Secondary Received Line Signal Detector | To DCE From DCE From DCE | Control |
**This signal is more commonly referred to as Data Carrier Detect (DCD).
Mechanical Interface Characteristics
The third area covered by RS-232 is the mechanical interface. Specifically, RS-232 specifies a 25-pin connector as the minimum connector size that can accommodate all the signals defined in the functional portion of the standard. The pin assignment for this connector is shown in Figure 2 . The connector for DCE equipment is male for the connector housing and female for the connection pins. Likewise, the DTE connector is a female housing with male connection pins. Although RS-232 specifies a 25-position connector, this connector is often not used. Most applications do not require all the defined signals, so a 25-pin connector is larger than necessary. Consequently, other connector types are commonly used. Perhaps the most popular connector is the 9-position DB9S connector, also illustrated in Figure 2. This 9-position connector provides, for example, the means to transmit and receive the necessary signals for modem applications. This type pf application will be discussed in greater detail later.
Evolution of RS-232 IC Design
Regulated Charge Pumps
The original MAX232 Driver/Receiver and its related parts simply doubled and inverted the input voltage to supply the RS-232 driver circuitry. This design enabled much more voltage than actually required; it wasted power. The EIA-232 levels are defined as ±5V into 5kΩ. With a new low-dropout output stage, Maxim introduced RS-232 transceivers with internal charge pumps that provided regulated ±5.5V outputs. This design allows the transmitter outputs to maintain RS-232-compatible levels with a minimum amount of supply current.Low-Voltage Operation
The reduced output voltages of the new regulated charge pumps and low-dropout transmitters allow use of reduced supply voltages. Most of Maxim's recent RS-232 transceivers operate with supply voltages down to +3.0V.AutoShutdown™
In the never-ending battle to extend battery life, Maxim pioneered a technique called auto-shutdown. When the device is not detecting valid RS-232 activity, it enters a low-power shutdown mode. An RS-232-valid output indicates to the system processor whether an active RS-232 port is connected at the other end of the cable. The MAX3212 goes one step further: it includes a transition-detect circuit whose latched output, applied as an interrupt, can awaken the system when a change of state occurs on any incoming line.AutoShutdown Plus™
Building on the success of AutoShutdown, devices with Maxim's AutoShutdown Plus capability achieve a 1µA supply current. These devices automatically enter a low-power shutdown mode either when the RS-232 cable is disconnected or the transmitters of the connected peripherals are inactive, or when the UART driving the transmitter inputs is inactive for more than 30 seconds. The devices turn on again when they sense a valid transition at any transmitter or receiver input. AutoShutdown Plus saves power without changes to the existing BIOS or operating system.MegaBaud
Moving beyond the EIA-232 specification is megabaud mode, which allows the driver slew rate to increase, thereby providing data rates up to 1Mbps. MegaBaud mode is useful for communication between high-speed peripherals such DSL or ISDN modems over short distances.High ESD
Some ICs are designed to provide high ESD protection. These ICs specify and achieve ±15kV ESD protection using both the human body model and the IEC 801-2 air-gap discharge method. Maxim's high-ESD protection eliminates the need for costly external protection devices such as TransZorbs™, while preventing expensive field failures.Support Issues
Capacitor Selection
The charge pumps of Maxim RS-232 transceivers rely on capacitors to convert and store energy, so choosing these capacitors affects the circuit's overall performance. Although some data sheets indicate polarized capacitors in their typical application circuits, this information is shown only for a customer who wants to use polarized capacitors. In practice, ceramic capacitors work best for most Maxim RS-232 ICs.Choosing the ceramic capacitor is also important. Capacitor dielectric types of Z5U and Y5V are unacceptable because of their incredible voltage and temperature coefficients. Types X5R and X7R provide the necessary performance.
Unused Inputs
RS-232 receiver inputs contain an internal 5kΩ pull-down resistor. If this receiver input is unused, it can be left floating without causing any problems. The CMOS transmitter inputs are high-impedance and must be driven to valid logic levels for proper IC operation. If a transmitter input is unused, connect it to VCC or GND.Layout Guidelines
Maxim RS-232 ICs should be treated like DC-DC converters for layout purposes. The AC current flow must be analyzed for both the charge and discharge stages of the charge-pump cycle. To facilitate an easy and effective layout, Maxim conveniently places all the critical pins in close proximity to their external components.RS-232 Transceivers in Tiny Packages
Low-power RS-232 transceivers are available in space-saving chip-scale (UCSP), TQFN, and TSSOP packages. The MAX3243E in a 32-pin (7mm x 7mm) thin QFN package saves 20% board space over TSSOP solutions. The MAX3222E, also available in a 20-pin (5mm x 5mm) TQFN, improves and thus saves board space by 40%. Other transceiver part families packaged in a TQFN, the MAX3222E and MAX3232E with two drivers and two receivers and the MAX3221E with a single driver and single receiver, feature AutoShutdown capability to reduce the supply current to 1µA (See Table 3). These RS-232 transceivers are ideal for battery-powered equipment.The MAX3228E/MAX3229E family in a 30-bump (3mm x 2.5mm) UCSP package saves about 70% board space, making these ICs ideal for space-constrained applications such as notebook, cell phone, and handheld equipment. Low-power RS-232 transceivers in space-saving UCSP with a low 1µA shutdown supply current are ideal for ultra-low-power system applications.
Table 3. RS232 Transceivers in Space-Saving Packages
| Part | Package | Shutdown Supply Current (µA) | Data Rate (kbps) | No. of Drivers/Receivers | ESD Protection (±kV) |
| MAX3221E | 20-Pin TQFN | 1 | 250 | 1/1 | 15 |
| MAX3222E | 16-Pin TQFN | 1 | 250 | 2/2 | 15 |
| MAX3223E | 20-Pin TQFN | 1 | 250 | 2/2 | 15 |
| MAX3230E | 20-Bump UCSP | 1 | 250 | 2/2 | 15 |
| MAX3231E | 20-Bump UCSP | 1 | 250 | 1/1 | 15 |
| MAX3232E | 16-Pin TQFN | 1 | 250 | 2/2 | 15 |
| MAX3237E | 28-Pin SSOP | 10nA | 1Mbps | 5/3 | 15 |
| MAX3243E | 32-Pin TQFN | 1 | 250 | 3/5 | 15 |
| MAX3246E | 36-Bump UCSP | 1 | 250 | 3/5 |
RS-232 Application Limitations
In the more than four decades since the RS-232 standard was introduced, the electronics industry has changed immensely. There are, therefore, some limitations in the RS-232 standard. One limitation—the fact that over twenty signals have been defined by the standard—has already been addressed. Designers simply do not use all the signals or the 25-pin connector.Other limitations in the standard are not necessarily as easy to correct.
Generation of RS-232 Voltage Levels
As explained in the Electrical Characteristics section, RS-232 does not use the conventional 0 and 5V levels implemented in TTL and CMOS designs. Drivers have to supply +5V to +15V for a logic 0 and -5V to -15V for a logic 1. This means that extra power supplies are needed to drive the RS-232 voltage levels. Typically, a +12V and a -12V power supply are used to drive the RS-232 outputs. This is a great inconvenience for systems that have no other requirements for these power supplies. With this in mind, RS-232 products manufactured by Dallas Semiconductor have on-chip charge-pump circuits that generate the necessary voltage levels for RS-232 communication. The first charge pump essentially doubles the standard +5V power supply to provide the voltage level necessary for driving a logic 0. A second charge pump inverts this voltage and provides the voltage level necessary for driving a logic 1. These two charge pumps allow the RS-232 interface products to operate from a single +5V supply.Maximum Data Rate
Another limitation in the RS-232 standard is the maximum data rate. The standard defines a maximum data rate of 20kbps, which is unnecessarily slow for many of today's applications. RS-232 products manufactured by Dallas Semiconductor guarantee up to 250kbps and typically can communicate up to 350kbps. While providing a communication rate at this frequency, the devices still maintain a maximum 30V/ms maximum slew rate to reduce the likelihood of crosstalk between adjacent signals.Maximum Cable Length
As we have seen, the cable-length specification once included in the RS-232 standard has been replaced by a maximum load-capacitance specification of 2500pF. To determine the total length of cable allowed, one must determine the total line capacitance. Figure 6 shows a simple approximation for the total line capacitance of a conductor. As can be seen, the total capacitance is approximated by the sum of the mutual capacitance between the signal conductors and the conductor to shield capacitance (or stray capacitance in the case of unshielded cable).
Figure 6. Interface cable-capacitive model, per unit length.
As an example, assume that the user decided to use nonshielded cable when interconnecting the equipment. The mutual capacitance (Cm) of the cable is found in the cable's specifications to be 20pF per foot. Assuming that the receiver's input capacitance is 20pF, this leaves the user with 2480pF for the interconnecting cable. From the equation in Figure 6, the total capacitance per foot is 30pF. Dividing 2480pF by 30pF reveals that the maximum cable length is approximately 80 feet. If a longer cable length is required, the user must find a cable with a smaller mutual capacitance.
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