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Serial Communications

Serial Communications. Standards (Partly Excerpted from Simpl Primer) Cabling Configuration Protocol. Standards RS-232, RS-422, and RS-485.

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Serial Communications

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  1. Serial Communications Standards (Partly Excerpted from Simpl Primer) Cabling Configuration Protocol

  2. StandardsRS-232, RS-422, and RS-485 • 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.

  3. RS-232 • By far the most popular of the serial protocols • “Single Ended” meaning the signals are referenced to ground and therefore limited in cable length • Approx. 50’ maximum cable length • Many A.V., lighting and other devices implement RS-232

  4. RS-422 and RS-485 • 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-485

  5. 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

  6. 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

  7. 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.

  8. Minimal Implementations • Although the RS-232 standard uses up to 25 wires, as few as 2 may be used for 1 way communications, or as few as 3 for 2 way communications. The other pins are either looped back on themselves, or not implemented internally. • The minimum signals are: Ground, TX and RX • Both communicating devices must be satisfied with the cabling

  9. 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

  10. 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

  11. Configuration • Once the hardware is correctly connected, the data configuration must be determined and adhered to by both devices • Data rate (Baud) • Number of data bits (7 or 8) • Number of stop bits • Parity or no parity • Handshaking

  12. 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.

  13. 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

  14. Data formats • The device protocol may be listed in Hex, Decimal, Binary or ASCII. The programmer needs to ensure that the program sends appropriate data format.

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