This allows you to define the RTU Number of the outstation, you can enter any number from 1 to 65535. This number should be unique to each outstation if you are not using Identification Codes.

RTU Number to Modbus Address Relationship

The RTU Number is related to the Modbus address associated with the outstation, which is limited to 0 to 255. Standard Modbus addresses are in the range 1 to 247. The addresses 0 and 248 to 255 are normally reserved. The following table illustrates the relationship between the RTU Number and Modbus address:

RTU Number	Modbus Address	RTU Number	Modbus Address	RTU Number	Modbus Address	...	RTU Number	Modbus Address
-	-	256	0	512	0	...	65280	0
1	1	257	1	513	1	...	65281	1
2	2	258	2	514	2	...	65282	2
3	3	259	3	515	3	...	65283	3
4	4	260	4	516	4	...	65284	4
5	5	261	5	517	5	...	65285	5
...	...	...	...	...	...	...	...	...
247	247	503	247	759	247	...	65527	247
248	248	504	248	760	248	...	65528	248
249	249	505	249	761	249	...	65529	249
250	250	506	250	762	250	...	65530	250
251	251	507	251	763	251	...	65531	251
252	252	508	252	764	252	...	65532	252
253	253	509	253	765	253	...	65533	253
254	254	510	254	766	254	...	65534	254
255	255	511	255	767	255	...	65535	255

The rows highlighted in green indicate the range of Modbus addresses that are normally reserved, the corresponding RTU Numbers can be used if there is no need to adhere to the Modbus standard.

You can calculate the Modbus address from any RTU Number using the following equation, where N is the RTU Number:

N - (INT(N/256) x 256)

Each outstation that uses a particular multidrop connection must have a unique Modbus address, for example, when using radio communications. Similarly, each outstation that is connected to a particular Modbus gateway must have a unique Modbus address.

 

The following diagram illustrates how RTU Numbers can be assigned to generate valid Modbus addresses. All the outstations connected to the Modbus Gateway must have a unique and valid Modbus address to ensure they correctly communicate with the Modbus master (Geo SCADA Expert).

In the example shown above, all the unused RTU Numbers that generate the Modbus addresses 1, 2, 247 and 5 should not be used for any additional outstations when connected to this gateway.

You can check the assigned Modbus number and RTU Number using the View Status facility for the outstation.

The Modbus address is called the Unit Identifier in Modbus TCP.

This allows you to define a two part identification code for the outstation. Each field allows you to enter any number from 0 to 9999. If the outstation does not have an identification code, which can occur in smaller systems, then you should enter zero for both codes.

Example of Identification Code application

 

The following diagram illustrates how Identification Codes can be used. In this simple example the Identification Codes are used to label the zone in which the outstations are located, 1.1 = Zone A, 2.1 = Zone B. In each case the combination of the RTU Number and the ID Code is unique.

The combination of the RTU Number and the Identification Code together must be unique within the whole database.

This combo-box allows you to select the protocol used by the outstation. There are two available:

This combo-box allows you to select the Master Station Number that Geo SCADA Expert represents to the outstation. You can select one of four.

Each master station that communicates with a Sofrel outstation must use a different Master Station Number, otherwise the masters will interfere with each other.

Select this check box if you want Geo SCADA Expert to set the clock on the outstation. If the outstation is connected to more than one Master Station, only one Master Station should be used to set the outstation clock.

Select this check box to have Geo SCADA Expert automatically acknowledge any alarms in the outstation.

Clear this check box if alarms are to be acknowledged using a different method, or by another Master Station.

If password management is enabled in the outstation then enter the password used to remotely access the outstation. Leave the field blank if password management is not enabled. Refer to the SOFTOOLS User Guide for information on how authentication is configured.

Authentication is only supported in the LACBUS-RTU protocol, and only for PSTN, GSM and network connections. The field is grayed out otherwise. The password must be between 6 and 12 characters and can only contain ASCII characters.

Use the fields within the Keep Alive Poll section of the Form to configure if, and how often, Geo SCADA Expert sends a periodic request to poll the outstation.

Sofrel outstations do not maintain permanent online communications and therefore this option only applies when using Open Line (PSTN) or Open Connection (Network) actions to hold the connection to the outstation open.

Normally the connection is closed as soon as all data has been retrieved and/or commands issued (for example controls), this means there is no communication link to keep alive, and therefore this isn't applicable.

This is designed to stop a link from being disconnected due to periods of inactivity.

Use the Poll When Online check box to enable Geo SCADA Expert to poll the outstation while the connection remains open. This option only applies when using Open Line (PSTN) or Open Connection (Network) actions to hold the connection to the outstation open. If this check box is cleared you must use the Refresh pick action to read the latest data.

The Poll When Online check box is selected by default. This results in Geo SCADA Expert polling the outstation while the link remains open.