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

Standards RS-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.

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

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

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.

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

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 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

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

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.