Data Acquisition

With a growing need for a distributed logger/controller network, RS-485 interfaces havebeen added to the hardware. The RS-485 interface typically operates as a balanced twowire, half duplex and un-terminated network (see Chapter 6). However, the protocolsused for communications on the network are often proprietary, with different manufacturersusing undisclosed protocols, and error detection/correction methods, betweendevices. This does not alter the fact that communications to devices on the RS-485network still occur via a single logger/controller, known as the local device, whichcommunicates to the PC via the RS-232 interface.An RS-485 repeater is used where more than 32 stand-alone devices are required onone network. A further 32 devices may be connected for each repeater used.When logger/controllers are constantly logging data to memory, data gathered can be sentto the host PC via the communications interface at any convenient time before thememory becomes full. This allows great flexibility in obtaining the data from a standalonedevice or a network of devices. However, when operating in real time, that is, datais continuously returned to the host PC from a single stand-alone logger/controller or anetwork of logger/controllers, an important consideration is ‘can the volume of dataobtained, be transmitted over the serial communications link?’ This depends on a numberof factors:• Baud rate• The number of channels being scanned• How often the channels are scanned• Whether the device is stand-alone or part of a distributed networkConsider first, a stand-alone logger/controller connected to the host PC via the RS-232interface operating at 9600 baud. As we have seen previously, data sent over thecommunications interface is sent in a 10-bit frame consisting of 1 start bit, 8 data bits and1 stop bit. The time to transmit each byte of data at 9600 baud is 1.042 ms (t = 10 bits/9600).Therefore, to transmit the maximum amount of data at the required baud rate, themaximum time between each data byte, being ready to be sent, is 1.042 ms.Consider a logger/controller that is scanning 10 channels. If, for each channel, sevenbytes of data are sent (on average), plus there are another ten bytes for each scan of theinput channels, then the total number of bytes to be sent for each channel scan is 80 bytes.The maximum time each channel scan could take is 83.36 ms (80 bytes × 1.042 ms).Therefore, all channels could be scanned at the maximum rate of approximately 12 Hz(1/83.36 ms).This calculation assumes that there are no hardware factors, such as multiplexer settlingtime, input amplifier-settling time etc, which limit this rate even further. Irrespective ofthe performance limitations imposed by the stand-alone communications interface,logger/controllers are not designed for high-speed data measurement.When operating as part of a distributed network, the considerations that determine theperformance of the system are different. Despite RS-485 being an extremely reliable