RS-232, RS-422, and RS-485: Understanding Serial Communication Standards, Exams of Data Communication Systems and Computer Networks

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Standards

• (^) The terms RS-232 , RS-422 , and RS-485 all refer to physical standards for serial communication developed by the Electronic Industries Association (EIA).
• (^) The standards specify the electrical interface between equipment.
• (^) Any device that conforms to one of the standards above should be able to communicate with any other device conforming to the same standard.

RS-232, RS-422, and RS-

RS-422 and RS-

• (^) These are less often used
• (^) Most often found in instrumentation systems in industrial environments
• (^) “Double Ended” signal electronics means much greater distances
• (^) Up to 5000’ cable length
• (^) Cresnet is RS-

RS-232 Standard Cabling

• (^) DB-25 is full industry standard
• (^) DB-9 has become more popular over last several years due to compact size. It is a limited but normally adequate implementation
• (^) DTE is Data Terminal Equipment (Computer)
• (^) DCE is Data Communications Equipment (Modem)
• (^) DTE talks to DCE

Crossover

• (^) DTE should connect to DCE with straight through cable
• (^) DTE can connect to DTE and DCE can connect to DCE using a crossover cable

Handshaking

• (^) Handshaking is the process of ensuring that data not be transmitted when the receiver is not ready and to ensure error free transmission.
• (^) Handshaking can be either hardware or software
• (^) Handshaking may not be required if the amount of data is small.

Other Serial Standards

• (^) USB, Firewire, SCSI and ethernet are all more sophisticated serial standards.
• (^) To date, they are mostly limited to computer peripherals
• (^) Expect to see USB may be a control standard in the future
• (^) Ethernet is beginning to be used for control

For Crestron Systems

• (^) The COM ports can be configured for RS232, RS422 or RS485 with minimal or greater level implementation.
• (^) The I.R. ports can be used as 1 way minimally implemented RS232 with Ground and TX only.
• (^) Crestron cable database shows many cabling requirements for controlled devices

Configuration

• (^) The controlled device will have a default configuration and may be changeable
• (^) Normally, best practice is to leave the controlled device in default and program the controller (Crestron) to that standard.

Protocols

• (^) Each device to be controlled will have a protocol, which are the “rules”
• (^) Often, this is simply a table with the controllable commands and the data to be sent for that command. - (^) Example: POWER ON = 05(H) Power Off = 06(H)
• (^) The (H) in the example refers to HEX