Data Acquisition

6.2.5 Examples of RS-232 interfacesAlthough, the RS-232-C interface standard defines only a point-to-point connection on thecommunications link, there are circumstances when more than one device is required to beconnected to the PC. An example of this would be the connection of multiple digitaltransmitters to a PC that has only one RS-232-C standard communications port. Whilst theRS-232 interface is not designed to be used in a multi-drop system, it is possible to connectthe modules to the PC using a daisy-chain network.In a daisy-chain configuration, each transmit-signal line output is wired to the ‘receiveinput’ signal line of the next digital transmitter in the daisy chain. This wiring sequence mustbe followed until the output of the last digital transmitter in the chain is wired to the receiveinput of the host. For the daisy-chain network to work correctly all digital transmitters mustbe set to the same baud rate, they must be uniquely addressable and must be able to echo anydata received at the receive input to its transmit output. All characters transmitted by the hostcomputer are received by each digital transmitter in the chain, and then passed on until theinformation is echoed back to the host. Commands sent from the host are examined by alldigital transmitters. The device which is addressed, responds by transmitting its response onthe daisy-chain network, rippling through any other modules before reaching its destination atthe receive input of the host computer.The daisy-chain must be carefully implemented to avoid pitfalls inherent in its structure. Asthe daisy-chain is a series-connected structure, any break in the communications link or failureof any of the devices connected will make the network inoperable.Where the distance between the host computer and several remote devices is beyond thecapacity of a normal RS-232 communications link, the network shown in Figure 6.7 can beimplemented. The communications link at either end is still RS-232.6.2.6 Main features of the RS-232 interface standardThe following are some of the main features of equipment that use the EIA-232 interfacestandard:• Communication is point-to-point• They are suitable for serial, binary, digital, data communication (data is sent bitby bit in sequence)• Most EIA-232-C communications data is in the ASCII code, although that is notpart of the standard• Communication is asynchronous (fixed timing between data bits, but variabletime between character frames)• Communication is full-duplex (both directions simultaneously) with a single wirefor each direction and a common wire• Voltage signals are:1. Logic 1: –3 volts to –25 volts2. Logic 0: +3 volts to +25 volts• The communications signal voltages are ‘unbalanced’, making them moresusceptible to noise• They provide reliable communication up to about 15 m (50 ft)• Data rates of up to about 20 kbps are possible. In spite of its popularity andextensive use, it should be remembered that the EIA-232 interface standard wasoriginally developed for interfacing data terminals to modems. In the context ofmodem requirements, EIA-232 has several weaknesses, most of which havearisen as a result of the increased requirements for interfacing other devices such