Voltage Signal Output Level of NI 843x RS-232 and RS-485/RS-422 Cards

Publish Date: Feb 08, 2018 | 0 Ratings | 0.00 out of 5 | Print

Overview

This document provides information about the voltage output levels on the serial communication lines for the NI 8430, 8431, 8432, and 8433, RS-232 and RS-485/RS-422 cards.

Table of Contents

  1. RS-232 Devices
  2. RS-485/RS-422 Devices
  3. Additional Resources

1. RS-232 Devices


According to the RS-232 standard, valid signals are either in the range of +3 volts to +15 volts or -3 volts to -15 volts with respect to the ground/common pin. Consequently, the range between -3 volts and +3 volts is not a valid RS-232 signal level. The standard also specifies a maximum open-circuit voltage of 25 volts, which means that RS-232 drivers and receivers must be able to withstand any voltage level up to +/-25 volts.

In the case of NI 843x RS-232 devices, the voltage output signal level is of +/-5.5 volt logic. Moreover, +5.5 volts represents a logic low and -5.5 volts represents a logic high. These output levels cannot be changed. Just as the RS-232 standard specifies, NI 843x devices can accept any voltage level that is between -15 volts and -3 or between +3 volts and +15 volts as a valid signal level.

 

Back to Top

2. RS-485/RS-422 Devices


The signals defined in both the RS-485 and RS-422 standards are differential. For RS-485, common-mode signal levels between -7 volts and +12 volts are tolerated. The minimum voltage level for signal recognition at the receiver is +/- 200 mV. For RS-422, common-mode signal levels between -7 volts and +7 volts are tolerated and the minimum voltage level for signal recognition at the receiver is +/- 200 mV.

In the case of NI 843x RS-485/RS-422 devices, the voltage output signal levels are derived from two lines, TX+ and TX-. For logic high, TX+ is set to +5 volts and TX- is set to 0 volts. To transmit logic low, T+ is set to 0 volts and TX- is set to +5 volts. These output levels cannot be changed. Keep in mind that these cards can accept any common-mode voltages between -7 volts and -200 mV or between +200 mV and +12 volts as valid signals.  This means that if you have common-mode voltage on your lines within those ranges you will receive false readings.

The following table summarizes the voltage output levels of NI 843x RS-232 and RS-485/RS-422 devices.

 

Output Logic States

NI 843x RS-232 Devices

NI 843x RS-485/RS-422 Devices

High Logic

+5.5 Volts

TX+ set to +5 Volts
TX- set to 0 Volts

Low Logic

-5.5 Volts

TX+ set to 0 Volts
TX- set to +5 Volts

 

Back to Top

3. Additional Resources

Back to Top

Bookmark & Share


Ratings

Rate this document

Answered Your Question?
Yes No

Submit