Configuring Multidrop Remote I/O Devices

Multidropping remote I/O devices from the same remote modem enables Citect SCADA to communicate with each I/O device one after the other, using the same phone connection, rather than hanging up and re-dialing.

Although you can configure multidrop remote I/O devices using the Device Communications Wizard, we recommend that you do it manually. The wizard would create a new port for each I/O device. This would mean you couldn't have any more than 255 I/O devices.

1. Run the Device Communications Wizard to configure the first device.
2. Configure every other I/O device manually in the Topology activity (see Add an I/O Device).
3. To increase the efficiency and capacity of your system you can allocate the same port name to I/O devices with the same communication settings.

Note: If you are multi-dropping and you want to be able to dial in to the I/O server, use your I/O devices to issue the caller ID, not the modem. This is because using the modem to issue the ID will send the same ID no matter which I/O device the call is relevant to. This makes it difficult to identify the I/O device that triggered the call.

By using the I/O device to issue the ID, the I/O server will receive a unique caller ID for each I/O device. However, not every I/O device is capable of issuing caller IDs. If multi-dropping, use I/O devices that can issue caller IDs.

To set up a modem connected to your dial-up remote I/O devices:

You can connect multiple I/O devices to the same modem. This means Citect SCADA can communicate with these I/O Devices one after the other using the same phone connection, rather than hanging up and re-dialing. This will reduce the number of telephone calls and increase the speed and efficiency of communications.

1. Connect the modem to a PC with a telephony program installed (for example HyperTerminal or PhoneDialer). This is where you will configure the modem to answer calls from Citect SCADA and/or initiate calls.
2. If the modem is necessary to make calls to Citect SCADA, configure it to initiate the phone call to a pre-determined Citect SCADA I/O server dial-In type modem (following manufacturer instructions).

Depending on your hardware, either the modem or an intelligent PLC can be responsible for initiating calls to Citect SCADA and identifying the caller. Whichever is responsible needs to have a caller ID set. The caller ID can be any combination of alpha-numeric characters and/or the character '_' (underscore).

Some modems have dip-switch settings, and some have initiation strings which can include auto-dial-up numbers that are stored within the modem's non-volatile memory. Consult the manual provided with the modem for exact details.

You can use either the Device Communications Wizard or the I/O devices form to set the caller ID for an I/O device.

If multi-dropping off a single modem, use your I/O devices to issue the caller ID, not the modem. This is because using the modem to issue the ID will send the same ID no matter which I/O device the call is relevant to, making it difficult to identify which I/O device triggered the call.

By using the I/O device to issue the ID, the I/O server will receive a unique caller ID for each I/O device. However, not every I/O device is capable of issuing caller IDs. If you are multi-dropping, use I/O devices that can issue caller IDs.

3. Set the modem's Data bits, Parity, Stop bits, and Serial-Rate to match manufacturer specifications for communication with the I/O devices.

Some modems do not allow you to manipulate their communications settings via methods such as extended AT commands or dip switches. If this is the case, the only way of setting the necessary values is to communicate with the modem using the values (for example, via HyperTerminal). Once this is done, the modem remembers the last values used to communicate with its serial port.

4. Connect the modem to the I/O devices.

To configure a modem at the I/O server you need to set it up in Windows and then set it up in Citect SCADA. See Configure a Modem.

If every one of your I/O devices are the same, you only have to do this once for each modem. However, if your I/O devices talk using different communication specifications (data bitsGroup of binary digits (bits) used to represent a single character of data in asynchronous transmission., parityA communications error-checking procedure. The number of 1's must be the same (even or odd) for each group of bits transmitted without error., stop bitsThe number of bits that signals the end of a character in asynchronous transmission. The number is usually 1 or 2. Stop bits are required in asynchronous transmissions because the irregular time gaps between transmitted characters makes it impossible for the server or I/O device to determine when the next character should arrive., and serial-rate), your modem has to be able to talk using each of these details as well. To set this up, you have to create a modem in Windows and Citect SCADA for each specification. See Example Configurations for Modems at the I/O Server.

Published June 2018