Cheap PLC that supports RS232 control


Thread Starter

Ralph Wheeldon

I'm working on a simple information system displaying production rates on a line and have a number of serial devices (keypads) for user input hanging off a PC.

I'm using VB6 as the HMI with some operator input coming from the keyboard and mouse. I have 2 machines that I want to have sensors count parts going through and update the PC by sending count info to the PC based on a timer (so say every 30 seconds I send count A and Count B as a string).

I also want to be able to send a "reset counters" command back. The reason for the RS232 is just I want to do it cheaply (I already have a spare port). This plant is loaded with Allen Bradley and I do lots of DDE/OPC but I can no way justify the cost of an RsLinx package for such a simple stand alone app. A cheap I/O card would do the trick as well. Does anyone have any suggestions?

You might wish to consider using some cheap IO modules which you connect directly through Ethernet. An example would be one of the Advantech 6000 modules. I know that they have a counter module. There are some other similar products from other companies on the market as well.

With the Advantech modules, you can talk to them through Modbus/TCP, or through Advantech's own UDP ASCII protocol. Advantech's UPD ASCII protocol is fully documented and it's very easy to use (as long as you have a library to read and write Ethernet sockets). For Modbus/TCP, if you do a Google search you'll find free source code for it.

As for cheap PLCs with serial ports, the cheapest that I know if is the Koyo (Automation Direct) "Click" (< 70USD). You might also want to look at their Modbus/TCP field I/O as an alternative to Advantech's. A lot of other "shoe-box" PLCs have serial ports (e.g. Siemens S7-200). As to which of the options makes the most sense probably depends on the installation considerations in your application.

OPC drivers are expensive, but if you use an open protocol like Modbus/TCP, then you don't need OPC. If you do a Google search, you'll find free source code for it.