Choosing the Right RS-232 Tran

文章摘要：Compatible Versus CompliantTo be compliant with the RS-232 specification, transmitters are required to provide a ±5V output signal level. For RS-232 receivers to be compliant, their thresholds must be ±3V maximum. Therefore, these devices operate with a healthy ±2V noise margin. Thus it is possible

Choosing the Right RS-232 Tran,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com
Compatible Versus CompliantTo be compliant with the RS-232 specification, transmitters are required to provide a ±5V output signal level. For RS-232 receivers to be compliant, their thresholds must be ±3V maximum. Therefore, these devices operate with a healthy ±2V noise margin. Thus it is possible to erode a percentage of the margin and still maintain accurate data transmissions. Margins are reduced when transmitter output voltages span a narrower range. When transmitter outputs are a minimum of ±3.7V, they are considered to be compatible, instead of compliant RS-232 outputs. For example, if you power an RS-232 device that doesn't contain charge pumps (thus less expensive and smaller) with ±5V supplies, its output voltages cannot possibly reach ±5V. A device operated under these conditions with transmitter outputs of ±3.7V at minimum is said to be RS-232 compatible instead of compliant.

Enhanced ESD Protection

All RS-232 devices include ESD protection structures on their terminals to protect against electrostatic discharges encountered during handling and assembly. Several manufacturers offer devices with both standard and enhanced protection. Since the standard and the enhanced-protection parts are almost always available with the same pin configuration, adding devices with enhanced ESD protection typically does not require any modifications to an existing PC board.

Some applications require ±15kV ESD protection on both the RS-232 and the CMOS digital terminals. For example, in order to reduce cost and size, cell phones do not include the RS-232 transceiver in the phone itself. Instead CMOS-level signals are routed through the connector at the bottom of the phone. If the cell-phone owner wants to connect to the RS-232 port of a laptop, he or she will purchase a "data-lump" cable, which includes an RS-232 transceiver. In this situation, both sides of the RS-232 transmitters and receivers are brought out to connectors, exposing them to increased ESD risk. In such cases, it may be desirable to have extended ESD protection on both the RS-232 and CMOS sides of the transmitters and receivers. The MAX3237E, MAX3380E and MAX3381E are three examples of transceivers that are suited to this task.

Data Rates

The latest RS-232 specification calls for data rates of 20kbps or below. Despite this restriction, the data rates of most RS-232 transceivers far surpass that speed. In fact, today's RS-232 transceivers reach speeds up to 1Mbps. See Figure 2 for an example of a transceiver operating at 1Mbps. To reach such speeds, these devices violate another RS-232 specification, the maximum allowed transmitter-output slew rate of 30V/µs. The RS-232 specification requires that transmitter outputs do not exceed this slew rate, thus reducing the amount of noise generated by fast edges. The outputs of transceivers that operate at speeds of up to 1Mbps are also slew-rate limited, however at a slew rate that exceeds the RS-232 specification. The outputs of these transmitters emit noise at higher frequencies than do their slower-slew-rate counterparts. The faster slew rate of these devices also creates another concern, compatibility with slower devices. If a part capable of Megabit-per-second operation transmits data at high speeds, a transceiver capable of operating at the same speed must be present at the other end of the cable. If a slower speed device is instead used at the other end of the cable, operation at slower data rates will probably work, although not guaranteed. The MAX3227E, MAX3245E and MAX3225E are three examples of transceivers that are capable of 1Mbps transmission rates.

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