in automation, the programmable logic controllers (PLC's) used to be programmed using the Ladder technique (a series of virtual rungs made of parallel/series normally open/closed contacts and coils). this was implemented in order to facilitate the comprehension of the electricians who had to work with those industrial computers. thanks God, nowadays, the instrumentation & control technician can use a structured text programmation style that solves many problems encountered with the previous technology. structured text consists in nothing more than a similar synthax of the usual instructions provided in high-level languages such as Delphi, C++ or Visual Basic. so, the loops and conditions "if then else", "select case", "while do", "repeat until" and "for next" are used, and it allows to program with a better flexibility and structure. with the arrival of the industrial networking connected to the PLC's, a revolution has also been achieved: the possibility to download/upload programs from the same computer to different PLC'S; however, the risk to make mistes is higher and the consequences may be catastrophic: if you do not download the right adresse node to the desired PLC' then the transmission speeds may operate differently from the transmitter to the receiver, and this may freeze the whole system! the solution is to shutdown all the PLC'S, and download one by one the correct programs! imagine the production lost! 
i used to work with daisy chain and token ring networking and although you a very good overall understanding of the networking topology, it is not guaranteed that you will troubleshoot easily since the configuration is MONSTRUOUS! it is tedious because the procedure to configure properly is not logical at all: it is a culture that you have to assimilate: man! i hate this kind of things!
above is an overview of how can look like the devices connected to the industrial PLC network:
the software used is for Allen-Bradley's PLC'S: Rockwell's RS-LINX. on the higher part, you can see the components connected on the Ethernet network: it is the network used for the miscellaneous PC's and servers.
the 1756 CND/B card on the backplane provides the gate between Ethernet and the Control Logix industrial network, on which are installed the PLC'S! you have to accurately configure each device so that the communication between the terminals is optimal. you have to use the who's who option in order to see if the adress nodes are properly configured.
the device net networking part is not shown, but they provide the gate between the PLC's networking and the discrete components, like starters, ON/OFF power outputs. not shown too: the flex IO adapters that provide the link between the PLC's and the analog input/outputs, like motor drives, temperature/flow/level/pH/angular speed/viscosity/pressure transmitters.
Edited by greenback
/PAlogo_v2.gif "Progarchives.com Homepage")
programming enhancements
Topic Options
cobb wrote: