Pack all your read data into as small an area as possible in the PLC, so that the HMI or SCADA can get all the data it needs in as few requests as possible. IOServer will dynamically combine read requests for data from one area in the PLC into a single request to the PLC. e.g. If you are requested the following data from a Modbus PLC: 40001(word), 40009/12(digital), 40017(long), 40025(real), 40033[8](string), 40041(bcd), 40049(long bcd), 40057(word), 40065(word), 40073/11(digital), 40081(long), 40089(real), 40097[8](string), 40105(bcd), 40113[12](string). IOServer will issue a single read request to the PLC get all the above data.
When writing data to the PLC, IOServer can combine write requests into a single write request to the PLC as long as there are no gaps between the original write requests. So if you have a recipe in the PLC, make sure they are no gaps between them, otherwise they will be written to at different time e.g. If you are writing to 00001,00002,00003,00004 ... to 00800 in a Modbus PLC, IOServer will issue a single write request to the PLC.
If a HMI is gathering data from 16 PLCs over a single multi-dropped link. The data update time will be very slow because the HMI can only get data from one PLC as a time, for each PLC that is offline there may be addtional delay of a couple of seconds. They will be a 1600 percent improvement in update time if the HMI has a separate link to each of the PLC, because all the PLC will be polled in parallel. All you need is 16 COM ports in your PC.
If you have an ethernet card and your PC have no more spare slot or IRQ line, you might try Digiboard Etherlite (http://www.digi.com/solutions/termsrv/etherlite.shtml), they have a EtherLite EL-162 "Port Server with 16 RJ45-based RS-232/422/485 (independently switchable) serial ports, supporting 10baseT and 100baseT (autodetecting), power supply, and software driver. 115K baud max. Stackable.". If you have 128 PLC, connect 8 PLC to each port or get more modules. The PLC is not on ethernet, the serial port on each PLC thinks that they are directly connected to the master. This scheme turns a multi-dropped network into a star network with the master at the centre. The master then can poll all the slave simultaneously, instead of one at a time.
If you are gathering data from more than 3 Allen Bradley PLC via the DH+. You should see a performance increase if you connect to each PLC directly via the serial port with the DF1 protocol.
Let the OPC Server to all the work. The OPC Server will tell you whenever there is a change in the value or the quality of any subscribed item. This will also reduce network traffic and CPU usage in the client.
Last updated: 26 November 2001