Page 374 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

366 CHAPTER 13 Communications
5. Transmitter and receiver timing signals (providing timing information for the transmitter and receiver—which may operate at different baud rates)
6. Channel test signals
The connectors for the RS-232 standard are traditionally of the 9-pin or 25-pin variety, but any number of connector types may be used as long as the function protocols are closely followed. The functions within the connector require the cabling technician to examine the functions for proper wiring of the communications channel in order for the transmit (Tx) and receive (Rx), as well as other typical functions, to be matched. Refer to the manual of each component in order to assure wiring continuity.
The EIA232 standard is applicable to data rates of up to 20,000 bps (the typical upper limit is 19.2 kb, or “kilo baud”). Typical baud rate values are 300, 1200, 2400, 9600, and 19,200 baud. Also, a maximum length for an RS-232 link is not specified in the standard, but typical maximum lengths are 8 m. Some ROV manufacturers stretch their RS-232 lengths up to 75
100 m with sig- nal loss. As such, the RS-232 protocol is length-limited and is typically only used in short runs at the surface or through the vehicle’s telemetry system when communications are to/from a fiber- optic link (i.e., short cable run from the sensor or surface computer to the fiber-optic multiplexer). The RS-232 standard, however, has been essentially replaced by the universal serial bus (USB) standard for connecting to DCE.
13.4.3 RS-422/485
The RS-422/485 protocol is part of a set of standards using “differential signaling.” Differential sig- naling is a technique whereby complimentary information is sent electrically over two wire pairs (termed a “differential pair”). The signals are compared at the receiving end. External interference tends to affect both wire pairs simultaneously; therefore, the information is embedded in the differ- ence between the wires (thus canceling out the surrounding noise). This technique is used in the RS-422/485 standards along with other standards including USB, Ethernet over twisted pair, serial digital interface (SDI), high-definition multimedia interface (HDMI), and Firewire.
The RS-422 standard defines the signal level for extending the range of serial devices to up to 1500 m. Often times, an RS-422 device is used in conjunction with an RS-232 device in order to extend the effective range of the signal. The RS-422 standard allows for unidirectional/nonrevers- ible, terminated or nonterminated transmission lines, point to point, or multidrop. In other words, the RS-422 standard allows for multiple receivers while the RS-485 allows for multiple drivers/ “talkers” for use in balanced digital multipoint systems.
The RS-422 standard is used extensively with sensors over copper for the copper-based OCROV systems as a simple extender while the RS-485 protocol is used for two-way communica- tions with various sensors (such as single-beam sonar systems). A comparison of the RS-422 and RS-485 protocols is provided in Table 13.6.
13.4.4 Ethernet
The Ethernet is an ANSI/IEEE (as adopted by ISO/IEC (International Electrotechnical Commission) in standard 8802-3) standard and is covered in IEEE 802.3 (as amended). It is the most widely used