Page 375 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

13.4 Standard protocols 367
Table 13.6 Comparison of the RS-422 Versus RS-485 Standards
Parameter RS-422 RS-485
Physical media Network topology Max devices
Max distance Mode of operation Max baud rate Voltage levels Available signals
Connector types
Twisted pair Point-to-point/multidrop 10 (1 driver/10 receivers) 1500 m
Differential
100 kbs to 10 Mbps
66 V
Tx 1 , Tx 2 , Rx 1 , Rx 2 (full duplex)
Any
Twisted pair P2P/multidrop/multipoint
32
256 devices
1200 m
Differential
100 kbs to 10 Mbps
27 V to 112 V
Tx1 /Rx1 , Tx2 /Rx2 (half duplex) Tx1 , Tx2 , Rx1 , Rx2 (full duplex) Any
local area network (LAN) protocol worldwide due to its simplicity, reliability, and relatively low cost.
As mentioned in Section 13.2.2, the essence of Ethernet transmission in through sending of frames of information through a “medium access control” (MAC) protocol for directing data traffic. The theory behind the Ethernet relay system is within the OSI model physical layer whereby the individual units of data are switched from dealing with bytes to directing traffic on ever-increasing sizes (and numbers) of frames. The original Ethernet started with a data rate of 10 Mbps and has now grown to over 100 Gbps (and growing!).
Ethernet was originally designed for transmission over coaxial cables but quickly adapted to the more cost-effective and malleable medium of twisted pair. Ethernet can now be transmitted over copper or fiber with fiber optics, gaining the most ground in data rates due to its huge capacity.
The version of Ethernet is designated based upon the cable-carrying capacity and medium used. For instance, a 10Base-T1 line denotes 10 Mbps transmitting a baseband signal over two twisted pair. Speeds of 10 Mbps are considered standard Ethernet while 100 Mbps is “fast Ethernet” and
.1000 Mbps is considered “gigabit Ethernet.” Some other transmission standards are listed in Table 13.7.
Ethernet transmission over varying lengths of cable are distance limited without some type of signal amplifier. Check the standard being used in order to determine if your cable tether length is suitable for Ethernet transmission without degradation of the signal.
13.4.5 Universal serial bus
The USB standard has become ubiquitous for connecting and powering small devices of a broad range (from cameras to smart phones to gaming consoles—it has even replaced many power char- gers for low-powered devices) over very short distances (,5 m). The latest iteration of USB is 3.0 (or later). With its release in November 2008, the standard has a theoretical transmission speed of up to 5 Gbps. The typical usage of the USB in subsea operations is for connecting serial devices to computing equipment.