Ideally, I would like an ActiveX control to allow me to read/write data in the Modbus slave device. I need to use Modbus function codes 3 & 16. Can I do this with 2 modems, one at PC end and one at device end?

 

Have you decided upon your MODBUS device? If not, have a look at the webDAQ/100. This 32-channel data acquisition unit is mainly intended for use over TCP/IP networks, but it can be used through it's serial port. The webDAQ supports MODBUS and comes with an ActiveX component. It is a simple matter to connect a modem to it's serial port and dial into it. See "www.cec488.com":http://www.cec488.com . It is available from "www.audon.co.uk":http://www.audon.co.uk in the UK.

 

Look into In-Gear ModBus ActiveX tools. If you stick to ModBus ASCII and use identical modems at either end, I don not see why it would not work. As a matter of fact, the Modicon WEB site is full of ModBus modem application write-ups. If not
there, check "ModBus.org":http://www.modbus.org .

 

Yes, it could be implemented, for either binary (such as Modbus RTU) or ASCII (such as Modbus ASCII) protocol.

You just need to preset the Dial-Up Modem on the Modbus Slave as Auto Answer (ATS0=1). In case the Modbus Slave device has a dial-up capable feature then the preset is not required.

On the Modbus Master, you need to add a routine in your Modbus protocol scanner/driver for dial-up handshaking before start sending a Modbus
read/write queries/commands.

M. Harisman

 

There's a caveat though - The 3.5 char Modbus RTU delimiter can cause problems if you're using a modem that compresses data. Also most standard
Office PC modems are 10 bit, whereas to run Modbus RTU with parity you need 11 bits. Westermo do a DIN rail mounting industrial modem that solves these problems.

Tim Linnell