The electrical specifications of RS-232 complaint devices include non-timing related specifications such as signal levels, leakage currents, power, and input thresholds for the device. The following list includes common specifications and where to find them in TI RS-232 compliant transceivers.

• Supply Voltage (V_CC, V_SS*, V+*, V–*, and V_L*):

  Supply voltage is the power source for the transceiver; however, due to the large voltage swings possible in many RS-232 applications there are multiple supply voltage ratings that all mean slightly different things. The * denotes a pin that may or may not be present on an RS-232 device.

• V_CC is the positive power supply – this is generally 3.3 V – 5 V for most modern RS-232 devices (but can be up to 15 V depending on the device). V_CC has three main uses that it can occupy in a common RS-232 device. The first is providing the bias for the controller side and bus side pins directly allowing positive voltage swings to be generated- in these devices a negative supply pin - V_SS - is also included to provide a negative supply voltage to provide negative voltage swings. The second is bias the controller side pins and to act as a input voltage for the integrated charge pump to generate the bus voltage swings. Finally, V_CC can also be used to just feed the charge pump in devices with separate logic voltage supply pins available.
• V+ and V– are the charge pump output pins when seen on a device and in the data sheet they represent the voltage ratings of these pins. If these pins exist on the device, the pins are usually connected only to an external capacitor. These output pins have ratings on the absolute maximum and recommended operating conditions table as well – but no external voltage is applied to this pin. These supplies are used to provide drive strength to the bus pins.
• V_L is the power supply for separate logic pin supply voltages in devices with separate bus and logic supply. This power supply allows a lower control voltage to interact with the console side pins as the interface with low voltage (< 3.3 V) controllers (down to 1.8 V). This information is found in the same data sheet sections as the other supply voltage parameters.
• Supply Current (I_CC):

  Supply current or I_CC is the quiescent current of the device while active with no loading to give a baseline power consumption for the device. This specification is important to understand if power savings is important to the application. Supply current is usually found in the data sheet either under Electrical Specifications or Electrical Specifications-Power.

• Common Driver Electrical Characteristics (V_OH, V_OL, V_IH, V_IL, I_IL, I_IH, I_OS, R_O, and I_OZ):

  Many RS-232 devices separate most of the driver electrical characteristics into independent sections in the data sheet – except for V_IH and V_IL which is typically found in the recommended operating conditions.

  • V_IL and V_IH are the console-side input thresholds and stand for Voltage, Input Low Threshold and Voltage, Input High Threshold, respectively. These values are applied to enable, driver input, or special feature pins that can register a logic low or logic high state. If an input voltage with respect to device ground is ≥ than V_IH (min), the pin state is registered a logic high and if the input voltage with respect to device ground is ≤ V_IL (max), the pin state is registered a logic low. On many devices V_IL (max) and V_IH (min) are rated to 0.8 V and 2 V, respectively, but on devices with a separate logic supply voltage or devices without internal charge pumps, these values can deviate from the most common threshold levels. Holding the input voltage between the V_IL (max) and V_IH (min) thresholds can cause large shoot-through currents and can lead to oscillations at output due to LC parasitics on the power line for the device.
  • I_IL and I_IH are the resulting current draw into a console side pin when the voltage applied with respect to ground is a logic low for I_IL and logic high for I_IH. These specifications are used to help size pullup and pulldown resistors to provide proper default state operation.
  • I_OS is the output short-circuit current from a driver output to device ground. This value gives the maximum expected short-circuit current and potentially also provides a typical short-circuit current value. The designer must control the short-circuit duration to prevent too much power being dissipated over the device. Repeated power overages can lead to premature part failure. Typically only one output is allowed to be shorted at a time. See the specific device data sheet for more information.
  • R_O is the output resistance of the driver. Generally this is when the supply voltage is disconnected from the device. The minimum of this specification is usually 300 Ω but many devices have a typical value in the KΩ to MΩ range. This specification gives an idea on how the driver loads the bus when unpowered.
  • I_OZ, also called I_OFF, is the leakage current from the driver when the driver is disabled. This is the amount of current that can be sourced or sunk from the driver pin when disabled. This helps characterize the bus loading when the driver is inactive.
• Common Receiver Electrical Specifications (V_OH, V_OL, V_IT+, V_IT–, V_HYS, R_I, and I_OZ)
  • V_OH and V_OL are the output high-level voltage and output low-level voltage, respectively, on the console side pins. When an RS-232 device receives data on one of the RX bus pins, the corresponding output voltage on the console side is V_OH for a logic high and V_OL for a logic low. These values are used to provide accurate communication between the RS-232 transceiver and controller.
  • V_IT+ and V_IT– are the positive-going and negative-going input voltage thresholds, respectively, for the RS-232 device RX bus pins. These voltage thresholds function similar to the console-side logic input thresholds V_IH and V_IL, but are dependent on current state. If the device is receiving a logic low when the input signal crosses the V_IT+ (max) boundary, a logic high is read at the RX bus pin. Whereas the inverse is true of the V_IT– (min) boundary. These voltage thresholds are used to help determine how much attenuation the system is able to handle from driver to receiver.
  • V_HYS is the hysteresis voltage between the V_IT+ and V_IT– thresholds. This parameter is used to determine the noise margin allowed between the input thresholds to provide error-free communication between driver and receiver.
  • R_I is the input resistance of the bus-facing RX pins on the RS-232 device. R_I acts a resistor to ground. The RS-232 standard requires a minimum input resistance of 3 kΩ – with many devices having a possible range of input resistance of 3 kΩ to 7 kΩ; however, some devices can be higher than 7 kΩ.
  • I_OZ is the console-side receiver output pin leakage current. I_OZ is the same parameter as I_OZ on the driver, but specifies the console-side RX output.