Page 373 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

was a trade organization that is typically mentioned in parallel with the TIA, but it ceased operations in February 2011. The common communications standards mentioned below are from consensus standards promulgated by the TIA/EIA over the past few decades. These are “Recommended Standards” that are cataloged as “RS” followed by the standard number (e.g., RS-232) that were jointly adapted by the two organizations. These standards are widely used today for serial data com- munications in the subsea industry.
13.4.2 RS-232
The EIA232 standard (commonly referred to as “RS-232”) is actually a series of standards for serial binary single-ended data and control signals for linking data terminal equipment (DTE) and data circuit-terminating equipment (DCE). See Figure 13.36 for a graphical depiction of the link. The standard specifies signal voltage/timing/function, a protocol for information exchange and wiring/ pin-out for mechanical connectors.
If all devices exactly followed the RS-232 standard, all cables would be identical with no possi- bility that an incorrectly wired cable could be used. Unfortunately, the standard varies depending upon a number of factors.
Signal functions in the EIA232 (RS-232) standard can be subdivided into six categories:
1. Signal ground and shield
2. Primary communications channel (including flow control signals)
3. Secondary communications channel (for control of a remote modem, requests for retransmission
due to errors, and control over the setup of the primary channel)
4. Modem status and control signals (modem status for establishing voice or data communications
channel)
Computer
DTE
Serial connectors
Cable
Modem
DCE
13.4 Standard protocols 365
Tether
FIGURE 13.36
Typical RS-232 component link.