Serial Communications

1
Serial Communications

• Standards
• (Partly Excerpted from Simpl Primer)
• Cabling
• Configuration
• Protocol

2
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.

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
(No Transcript)
7
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

8
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.

9
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

10
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

11
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

12
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

13
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.

14
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

15
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.