What are the differences between the different serial protocols, RS232, RS423, RS422 and RS485?

• RS232 uses single ended TX and RX. This means a common ground wire is shared between TX and RX.
  • Only 3 wires are needed for a data-only serial channel: TX, RX, and GND.
  • Disadvantages of single ended signaling is that it is more susceptible to noise than differential signaling, effective cable distances are shorter and data rates are slower.
  • The voltage ranges of RS232 signaling is +3V to +15V for a "high" and -3V to -15V for a "low".
• RS423 uses single ended TX and differential RX. TX is comprised of 2 wires:
  • TX+, a single ended signal, and TX- (which is really just GND).
  • RX is comprised of RX+ and RX-, which go into a true differential receiver.
  • 4 wires are needed for a data-only serial channel: TX+, TX- (GND), RX+ and RX-.
    • The single ended TX+ pin drives the RX pin of an RS232 port
    • TX- (GND) just ties to the RS232 GND pin and provides a common GND.
  • The voltage levels of signaling is +5V for a "high" and -5V for a "low", which is within the RS232 voltage level ranges mentioned above.
• RS422 uses differential TX and RX.
  • The TX pair of wires TX+ and TX- either drive
    • TX+ to 5V and TX- to 0V for a "high", or
    • TX+ to 0V and TX- to 5V for a "low".
  • TX- is not tied to GND as in RS423, instead it drives "opposite" the voltage that is on the TX+ pin.
  • RS422 RX+ and RX- detect differential voltages just as in the RS423 case.
  • 4 wires are needed for a data only serial channel: TX+, TX-, RX+ and RX-.
  • Since the TX pair of wires does not have a GND wire, RS422/RS485 can not be connected directly to an RS232 port.
    • Attempting to do this (by connecting RS422 TX+ to RS232 RX and RS422 TX- to RS232 GND) would cause
      • the RS422 TX- pin to be held at GND and
      • the RX422 TX+ pin would drive 5V for "high" (which is valid RS232) and 0V for "low" (which is not a valid RS232 signal level).
  • Since RS422/RS485 are truly differential for both TX and RX, cable distances can be much longer and data rates much higher than RS232.
    • If twisted pair cabling is used, (which makes sense because you end up with a complementary TX+ and TX- pair and RX+ and RX- pair), you reduce EMI emissions because the wire pairs help to cancel external transmissions.
    • These wire pairs also reduce the impact of external electrical noise on the signaling.
• RS485 uses differential TX and RX as in RS422, however RS485 is a superset of RS422. It adds a slightly wider input voltage range on its RX pair, to make it less susceptible to noise and GND potential differences among RS485 devices.
  • RS485 also adds multidrop capability. There can be up to 32 RS485 devices on a single set of serial lines.
    • This is achieved by having an "output enable" on the TX ports and allowing only one RS485 device to transmit at a time on a given pair of wires.
    • No isolation transformer is needed as in Appletalk described below.
  • RS485 serial "networks" can be wired for 2 or 4 wire mode.
    • In 2-wire mode, only one RS485 device can transmit at a time, this is known as half-duplex communication. An advantage to this topology is that you can save on cabling cost since the cable can be several thousand feet in length.
    • In 4-wire mode, a single RS485 device is designated as the "master" device. It's TX pair is connected to all the other "slave" devices' RX pairs, and all the slave's TX pairs are tied to the master's RX pair.
      • This has the advantage of enabling the master device to transmit to the slaves while a single slave device may also transmit back to the master simultaneously. This is known as full-duplex communication.
      • Another advantage of this topology is that the slaves are unable to inadvertently transmit to each other's RX pairs because they are not physically connected. This may make it easier to run slaves of different protocols on the same serial network.
    • In 4-wire mode, the Device Server's serial port or screw terminal port can be connected directly to another RS422/RS485 port.
      • The RS485 port cannot be connected directly to an RS232 port without an RS485-to-RS232 adapter for reasons described above in the RS422 section.
    • Since, like RS422, RS485 is differential it has similar cable length, data rate and noise immunity characteristics to RS422 described above.
• Appletalk uses a variation of RS422.
  • TX+ and TX- are "turned off" into a high impedance state (Tristate) where they are not driving a valid differential voltage.
  • By doing this they enable multidrop capabilities where multiple parties can share a common set of serial lines.
  • This is somewhat similar to RS485 described above, however Appletalk typically also includes an isolation transformer between the RS422 driver and the serial cable.
  • There are no Lantronix products that support Appletalk

[Originally Published On: 06/29/1999 10:54 AM]