AN00004.TXT Advanced Communication Board Developer's Toolkit 1995 ---------------------------------------------------------------------- ---------------------------------------------------------------------- Subject: Differences Between Electrical / Mechanical Interfaces. This application note covers the differences between several common interface specifications that are used in communications. Each section is dedicated to a different specification or standard. If there are any questions or if additional information is required, please contact technical support at the numbers listed in the README.TXT file located in the root directory of this diskette. 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-AT 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 Kilo bits per second 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 +8 to +10 volts) represents a binary 0 and -12 volts (-8 to 10 volts) denotes a binary 1. RS-422 RS-422, unlike RS-232 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 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 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-485 This interface is very similar to RS-422 in several ways. RS-485 is a differential interface that 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. Two wire mode requires that data be transferred in only one direction at a time and the two transmit pins should be connected to the two receive pins (TX+ to RX+ and TX- to RX-). Four wire mode will allow 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 AND RS-449 RS-530 and RS-449 (a.k.a. EIA-530 and EIA-449) are similar to RS-422 and RS-485 because they are differential interfaces, but these two standards provide a specified pin-out that defines a full set of modem control signals that can be used for regulating flow control and line status. RS-449 is defined on a standard 37 pin D sub connector while RS-530 is backward compatible and is replacing RS-449. RS-530 is defined on a 25 pin D sub connector. These two interfaces define RS-422 as an electrical specification. MIL-188 This communications standard has two interface options, 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 a unbalanced or single ended interface. Both MIL-188 interfaces are implemented on a RS-530 connector. MIL-188 / C and MIL-188 / 114 have signal levels from +6 volts to -6 volts and are ideal for long distance communications at high speeds. CCITT V.35 V.35 is a standard defined by CCITT that specifies an electrical, mechanical and physical interface that is used extensively by high-speed digital carriers such as AT&T Dataphone Digital Service (DDS). CCITT V.35 is an international standard that is often refereed to as "Data Transmission at 48 Kbps Using 60 - 108 KHz Group-Band Circuits." CCITT V.35 electrical characteristics are a combination of unbalanced voltage and balanced current mode signals. Data and clock signals are balanced current mode circuits. These circuits typically have voltage levels from 0.5 Volts to -0.5 Volts (1 Volt differential). The modem control signals are unbalanced signals and are compatible with RS-232. The physical connector is a 34 pin connector that supports 24 data, clock and control signals. The physical connector is defined in the ISO-2593 standard. CCITT is implemented by both DTE and DCE interfaces. CURRENT LOOP This communication specification is based on the magnitude and direction 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 effective for data rates up to 19.2 Kilo baud. 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. Note that there is not an ACB adapter that supports the Current Loop interface.