Data Acquisition

interface, even at high speed, the potential speed at which the network can operate islimited by a number of factors:• Each device in the system has a unique address and must be polled by the hostcomputer for information.• Only one device can be polled at any time.• As the RS-485 network is half-duplex, the host PC must wait for a response toeach request for data before polling the next device.• There is an inherent delay, or turnaround time, in responding to a host requestirrespective of whether one byte or one hundred bytes of data are returned inthe response. This is because the device must interpret, process, then act onthe command received before returning its response.• Where the RS-485 network is operating much faster than the RS-232 interface(i.e. more than twice as fast) the potential speed at which the data can be sentto the host PC is limited by the baud rate of the RS-232 interface. Where thisis not the case, the baud rate of the RS-485 network is the limiting factor.It has been shown in the example for a stand-alone device, that a device scanning 10input channels and returning 80 bytes of data to the host PC for each scan, would take83.36 ms to transmit the data. If the time required to transmit a ten-byte poll command is10.42 ms, and assuming a turnaround time of 1 sec, the total time to obtain the data froma single device is 1093.78 ms. The time taken for 10 devices operating in the samemanner would be approximately 11 sec. Clearly, the system would not be able to operatein real time unless each device scanned its input channels and returned data to the hostonce every 11 sec.Where the channels of one or more of the devices must be scanned at a faster rate, thedata should be logged to memory and returned at a more convenient time.Data loggers have traditionally used RS-485 as a type of networking system. RS-485works very well for multi-dropping up to 32 data loggers. As requirements have expandedin the plant environment, we have seen a need to connect data loggers toexpanding networks of systems. This has seen the rise of data loggers being connected viaexisting Ethernet networks. The advantages are obvious. One of the main problems withconnecting data loggers on an RS-495 network is the limited range of access to thesystem. By having access to the data loggers over the Ethernet, the user can view andeven change data anywhere the network is connected. This brings rise to the use of dataloggers using the Internet. Intra- and inter-networking of data loggers also raises securityissues. How safe is the data? Will someone that does not have authorization be able toaccess the hardware? These problems and their solutions will be the subject of muchdiscussion when it comes to connecting Ethernet to data loggers.7.5 Stand-alone logger/controller firmwareThe hardware represented by stand-alone, or distributed loggers/controllers, and used as apart of a data acquisition system, provides the physical interface, which allows the PC tomeasure data from and control real-world signals. The software that is stored andexecuted from the ROM or EPROM of the stand-alone device, and known as thefirmware, controls the continuous operation of the stand-alone device. However, thefirmware does not initiate any data acquisition and control functions by itself. Instead, the