Overview of Serial Communication Standards
Asynchronous Communications Overview
Asynchronous communications is the standard means of serial data communication for PC compatibles and PS/2 computers. Serial data communications implies that individual bits of a character are transmitted consecutively to a receiver that assembles the bits back into a character. Data rate, error checking, handshaking, and character framing (start and stop bits) are predefined and must correspond at both the transmitting and receiving ends. Serial asynchronous communications is typically implemented with a Recommended Standard (RS). The standard usually defines signal levels, maximum bandwidth, connector pin-out, supported handshaking signals, drive capabilities, and electrical characteristics of the serial lines. The following sections briefly describe some of the more common communication standards. Voltage levels that are stated are typical and may vary due to line characteristics. All interfaces accept a range of acceptable electrical and physical parameters and may even operate in excess to the specified standard under certain line characteristics. The full specification for each standard is available from almost any dealer of engineering documents. For a more detailed explanation of asynchronous serial communications, Sealevel technical support recommends the book Technical Aspects of Data Communications by John E. McNamara, published by Digital Press (DEC) 1982.
Synchronous Communications Overview
“Synchronous Communications is used for applications that require higher data rates and greater error checking procedures. Character synchronization and bit duration are handled differently than asynchronous communications. Bit duration in synchronous communications is not necessarily predefined at both the transmitting and receiving ends. Typically, a clock signal is provided in addition to the data signal. This clock signal will mark the beginning of a bit cell on a predefined transmission. The source of the clock is predetermined and sometimes multiple clock signals are available. For example, if two nodes want to establish synchronous communications, point A could supply a clock to point B that would define all bit boundaries that A transmitted to B. Point B could also supply a clock to point A that would correspond to the data that A received from B. This example demonstrates how communications could take place between two nodes at completely different data rates.
Character synchronization with synchronous communications is also very different from the asynchronous method of using start and stop bits to define the beginning and end of a character. When using synchronous communications, a predefined character or sequence of characters is used to let the receiving end know when to start character assembly. This predefined character is called a sync character or sync flag. Once the sync flag is received, the communications device will start character assembly. Sync characters are typically transmitted while the communications line is idle or immediately before a block of information is transmitted. To illustrate with an example, let’s assume that we are communicating using eight bits per character. Point A is receiving a clock from point B and sampling the receive data pin on every upward clock transition. Once point A receives the pre-defined bit pattern (sync flag), the next eight bits are assembled into a valid character. The following eight bits are also assembled into a
character. This assembly will repeat until another pre-defined sequence of bits is received (either another sync flag or a bit combination that signals the end of the text, i.e., EOT). The actual sync flag and protocol varies depending on the sync format (SDLC, BISYNC, etc.).
Serial communications, synchronous or asynchronous is typically implemented with a Recommended Standard (RS). In most cases, the standard is set by the Electronic Industries Association (EIA). The standard usually defines signal levels, maximum bandwidth, connector pin-out, supported handshaking signals, drive capabilities, and electrical characteristics of the serial lines. The following section briefly describes some of the more common communication standards. Voltage levels stated are typical and may vary due to line characteristics. All interfaces accept a range of acceptable electrical and physical parameters and may even operate in excess to the specified standard under certain line characteristics. The full specification for each standard is available from a number of engineering document dealers. For a detailed explanation of serial communications, please refer to the book Technical Aspects of Data Communications by John E. McNamara, published by Digital Press (DEC) 1982.”
Electrical Interface Standards
RS-232
Probably the most widely used communication standard is RS-232. This implementation has been defined and revised several times and is often referred to as RS-232C or EIA-232. The most common implementation of RS-232 is on a standard 25 pin D sub connector, although the IBM PC computer defined the RS-232 port on a 9 pin D sub connector. Both implementations are in wide spread use. RS-232 is capable of operating at data rates up to 20 Kbps / 50 ft. The absolute maximum data rate may vary due to line conditions and cable lengths. RS-232 often operates at 38.4 Kbps over very short distances. The voltage levels defined by RS-232 range from -12 to +12 volts. RS-232 is a single ended interface, meaning that a single electrical signal is compared to a common signal (ground) to determine binary logic states. A voltage of +12 volts (usually +3 to +10 volts) represents a binary 0 and -12 volts (-3 to -10 volts) denotes a binary 1.
RS-422
The RS-422 specification defines the electrical characteristics of balanced voltage digital interface circuits. RS-422 is a differential interface that defines voltage levels and driver / receiver electrical specifications. On a differential interface, logic levels are defined by the difference in voltage between a pair of outputs or inputs. In contrast, a single ended interface, for example RS-232, defines the logic levels as the difference in voltage between a single signal and a common ground connection. Differential interfaces are typically more immune to noise or voltage spikes that may occur on the communication lines. Differential interfaces also have greater drive capabilities that allow for longer cable lengths. RS-422 is rated up to 10 Megabits per second and can have cabling 4000 feet long. RS-422 also defines driver and receiver electrical characteristics that will allow 1 driver and up to 32 receivers on the line at once. RS-422 signal levels range from 0 to +5 volts. RS-422 does not define a physical connector.
RS-423
The RS-423 specification defines the electrical characteristics of unbalanced voltage digital interface circuits. The voltage levels defined by RS-423 range from -5 to +5 volts. RS-423 is a single ended interface, meaning that a single electrical signal is compared to a common signal (ground) to determine binary logic states. A voltage of +5 volts represents a binary 0 and -5 volts denotes a binary 1. RS-423 is rated up to 100K bits per second. RS-423 defines driver and receiver electrical characteristics. RS-423 does not define a physical connector.
RS-449
RS-449 (a.k.a. EIA-449) compatibility means that RS-422 signal levels are met, and the pin-out for the DB-37 connector is specified. The EIA (Electronic Industry Association) created the RS-449 specification to detail the pin-out, and define a full set of modem control signals that can be used for regulating flow control and line status.
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for partyline or multi-drop applications. The output of the RS-422/485 driver is capable of being Active (enabled) or Tri-State (disabled). This capability allows multiple ports to be connected in a multi-drop bus and selectively polled. RS-485 allows cable lengths up to 4000 feet and data rates up to 10 Megabits per second. The signal levels for RS-485 are the same as those defined by RS-422. RS-485 has electrical characteristics that allow for 32 drivers and 32 receivers to be connected to one line. This interface is ideal for multi-drop or network environments. RS-485 tri-state driver (not dual-state) will allow the electrical presence of the driver to be removed from the line. The driver is in a tri-state or high impedance condition when this occurs. Only one driver may be active at a time and the other driver(s) must be tri-stated. The output modem control signal Request to Send (RTS) controls the state of the driver. Some communication software packages refer to RS-485 as RTS enable or RTS block mode transfer. RS-485 can be cabled in two ways, two wire and four wire mode. Two wire mode does not allow for full duplex communication, and requires that data be transferred in only one direction at a time. For half-duplex operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx- to Rx-). Four wire mode allows full duplex data transfers. RS-485 does not define a connector pin-out or a set of modem control signals. RS-485 does not define a physical connector.
RS-530
RS-530 (a.k.a. EIA-530) compatibility means that RS-422 signal levels are met, and the pin-out for the DB-25 connector is specified. The EIA (Electronic Industry Association) created the RS-530 specification to detail the pin-out, and define a full set of modem control signals that can be used for regulating flow control and line status. The RS-530 specification defines two types of interface circuits, Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The Sealevel Systems adapter is a DTE interface.
Current Loop
This communication specification is based on the absence or presence of current, not voltage levels, over the communication lines. The logic of a Current Loop communications circuit is determined by the presence or absence of current (typically + or – 20mA). When referring to the
specification, the current value is usually stated (i.e. 20mA Current Loop). Current Loop is used for point to point communication and there are typically two current sources, one for transmit and one for receive. These two current sources may be located at either end of the communication line. To ensure a proper current path to ground, or loop, the cabling of two current loop communication ports will depend on the location of the current sources. Current Loop is normally good for data rates up to 19.2 Kbps. This limitation is due to the fact that the drivers and receivers are usually optically isolated circuits that are inherently slower than non-isolated equivalent circuits.
MIL-188
This communications standard comes is two varieties, MIL-188/C and MIL-188/114. Both of these interfaces are military standards that are defined by the US Department of Defense. MIL-188/114 is a differential interface and MIL-188/C is an unbalanced or single ended interface. Both MIL-188 interfaces are implemented on an RS-530 connector. MIL-188/C and MIL-188/114 have signal levels from +6 volts to -6 volts and are ideal for long distances at high speeds.