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The ValuPoint can be a Modbus TCP client and server. The terms client and server are more often used with Ethernet network devices, but for Modbus purposes, they still mean master and slave respectively. You must choose one or the other between master and slave for Modbus RTU, but Modbus TCP can be both simultaneously thanks to Ethernet.
As a master (client) you can read Modbus data from, or write Modbus data to, other Modbus TCP devices. The ValuPoint will periodically poll the other Modbus devices according to register maps you set up. To read from a remote Modbus device, configure a Read Map. To write to a remote Modbus device, configure a Write Map.
Data read from a remote device is stored in a local register when received. Data written to a remote device is taken from a local register when sent. The local registers are the same collection of registers that are accessible to other Modbus TCP clients as a collection of holding registers. These local registers are also accessible to the SNMP MIB.
The following section details the process of setting up read and write maps via the web interface. Once familiar with map content, you have the option of importing bulk configurations as a CSV file. The import function is found on the File Manager page, and details about the content and format of the CSV file are found in Appendix B.
The Modbus TCP client is the Ethernet version of Modbus master. Modbus RTU requires a slave address. Modbus TCP also requires, in effect, a slave address. In the case of TCP, that slave address is an IP address. Since entering the IP address requires more effort than one simple slave number, and because internally a network socket is required per IP address, the TCP devices are set up in their own table.
For each Modbus TCP device that you wish to read and write, enter its IP address on the TCP Setup Devices page. The port is normally going to be 502 (the standard Modbus TCP port), but if it is different, enter that number here.
A unit number is always included in each Modbus TCP packet. This is the equivalent of the RTU slave address. Some TCP devices pay no attention to unit and simply echo back whatever you had sent. However, if you are accessing RTU devices on the other side of a TCP to RTU router (gateway), then the unit does become the RTU slave address on the RTU side of the gateway and multiple RTU devices are accessed at the same TCP IP address.
If "Unit" on the devices page is left at zero, then unit #1 is used by default. Otherwise the number entered here becomes the default. However, each individual read and write map can have different unit numbers, as would be required for accessing multiple RTU devices via a gateway at one IP address. When a non-zero unit number is placed in the read or write map, it will override the default on the Devices page.
If coil and holding register writes must be done with write single function codes 5 and 6 rather than write multiple function codes 15 and 16, then check the "Use FC 5/6" box for this device. The ValuPoint will default to write multiple.
The default poll rate entered here will be used for all Modbus TCP Read and Write maps unless a different number is entered in the expanded view of the map.
The static IP address for the Modbus TCP device can be either IPv4 or IPv6. Along with selecting the desired IP version, be sure to enter the applicable IP address format. In addition, if IPv6 is used, be sure IPv6 is enabled on the Network configuration page.
You have the option of referring to a Modbus TCP device by domain name. If you use a domain name, be sure that domain can be found at the DNS servers provided on the Network setup page. If found, the IP address provided by DNS will be displayed here (but not saved in the XML configuration file).
Getting the ValuPoint to read registers from another Modbus device requires setting up a "Read Map" as shown here.
Map number simply tells you where you're at on the list of register maps. Click "next" and "prev" to scroll through the list. To advance directly to a specific map, enter the desired number in the "Showing" box, then click Update.
Maps entered on this page only read data from remote devices. Go to the TCP Write Map to write data to those devices. The full parameter set is different for read versus write.
To create a Read Map, start by selecting the Modbus register type to read from the drop-down list.
Select the data format expected in the remote Modbus register. The abbreviation INT under Format means signed integer, while UINT represents unsigned integer. The INT and UINT are followed by the number of bits to be read (which translates into 1, 2, or 4 consecutive holding registers). The FLOAT format refers to 32-bit IEEE 754 format while DOUBLE refers to 64-bit IEEE 754 floating point. The MOD10 format is unique to Schneider Electric power meters, and is supported in 2, 3, and 4-register formats. (Note: Use INT-16 or UINT-16 for coils or discrete inputs - in this case format only affects local register data conversion.)
Enter the register number to read from the remote TCP server. Do not use Modicon numbers here. In other words, if your device's documentation says read register 40001, that is short hand (Modicon notation) for saying read holding register 1. Refer to the Modbus Reference Information section of this user guide for more discussion about register numbers like 40001. If you enter 40001 here to read the first holding register, you will get an exception error since the actual register number is not 40001.
Select a TCP device from the list that this register should be read from. Only devices entered on the Devices page will appear in the list.
The Local Register is where data read from the remote Modbus TCP server will be stored locally in the ValuPoint. If the local register data format does not match what you are reading from the Modbus device, the data will be converted automatically when it is read.
Click on the Map number in the first column to access the expanded view of the Read Map.
For each remote register to be read, select the register type, format, number. Select a TCP server device from the list to read from. Only devices entered on the Devices page will appear here. If a unit number other than the default unit entered for this TCP server on the Devices page should be used, enter that unit number here.
The optional bit mask and scaling are discussed with examples below.
Modbus protocol treats all input registers or holding registers as strictly 16-bit registers. To accommodate 32-bit or longer data, Modbus devices use multiple consecutive "registers" to hold the data. There is no standardization of whether the least significant part of the data comes first or last. Therefore, ValuPoint lets you set that according to whatever the remote Modbus device requires. If the least significant data is found in the first (or lower numbered) register in your Modbus device, then check the box after "With low register first".
The poll rate ("Repeat this process...") determines how often the remote register will be read. If zero is entered here, the rate will become the default poll rate given on the Devices page for the Modbus TCP device selected.
The default value will be stored into the local register after the given number of read failures if the fail count is non-zero. Setting the count to zero will disable the default, and the object will retain the most recent value obtained. If the default value does take effect, the actual data value read will be retained when communications are restored.
You have the option of making this Read Map conditional. If an index register number is provided and the Enable box is checked, then this read map will only be executed when the index register (local register) contains the value given. This allows multiple read maps to supply data to the same local register based on the value of the index register. It also allows reading to simply be suspended if a single read map supplies data to the local register. In a more sophisticated scenario, you could potentially suspend reading of the remote Modbus device if you know the device is powered down.
Map number simply tells you where you're at on the list of register maps. Click "next" and "prev" to scroll through the list. To advance directly to a specific map, enter the desired number in the "Map #" box, then click Update.
Delete will remove the map number shown in the "Map #" box. Insert will insert a new map before the map number shown, and is used for placing maps between existing maps. It is not necessary to use Insert to add maps to the bottom of the list or to define any map presently having zero for a source object or "none" for remote type.
Selecting "none" for remote type effectively deletes the map even though it will still appear in the list until deleted. Unused maps at the end of the list will always show none as the type. If you wish to prevent these from being displayed, reduce the number of maps enabled.
The number of maps enabled simply limits the scope of map review so that you do not have to review a lot of unused maps. If the displayed maps are used up and you need more, increase the enabled number.
You have the option of providing a scale and offset. A scale of zero will cause scale and offset to be ignored. If provided, the Modbus data will be treated as raw data. When the Modbus data is received, it will be multiplied by scale, then added to offset, and then stored in the local register. If the Modbus device was providing degrees Celsius, and the scale factors illustrated above were used, then a Modbus value of 25 would result in the local register receiving a value of 77 (degrees Fahrenheit).
It is common for Modbus devices to pack a number of status bits into a single holding register. In order to do meaningful things based on a single bit, it is sometimes necessary to split that register into multiple local registers. ValuPoint supports this requirement by providing an optional bit mask.
If a bit mask is entered (in hexadecimal), and the remote register type is signed or unsigned integer (16-bit or 32-bit data), the mask will be bit-wise logical AND-ed with the Modbus data, and the retained bits will be right justified in the result stored locally. Refer to the Modbus Reference section in this user guide for a list of all possible mask values.
If the read maps referencing the same remote register are created in sequential contiguous order, the ValuPoint will optimize the TCP activity by reading the remote register once and then sharing the data with all of the read maps in the group. The example illustrated for Modbus RTU in section 5.2 works exactly the same for TCP. In that example, four consecutive read maps reference the same remote register, each selecting a different bit. The first map is selecting bit 0, the second selecting bit 1, and so on.
Getting the ValuPoint to write registers to another Modbus device requires setting up a "Write Map" as shown here. Much of the Write Map is configured the same as a Read Map.
The data direction is reversed but the same selections are still made. Select the local register that will be the source of data to write to the remote Modbus TCP device. Select the register type, data format, and register number to be written to in that device. Select a TCP server device from the list to write to. Only devices entered on the Devices page will appear here. If a unit number other than the default unit entered for this TCP server on the Devices page should be used, enter that unit number here. Click on the map number in the first column to access additional optional configuration parameters.
The local register data may be written to the remote device periodically, or when it changes, or both. To send upon change (send on delta), check the first box and enter the amount by which the local register must change before being written to the remote device. To guarantee that the remote register will be written at least occasionally even if the data does not change, check the second box and enter some amount of time. This time period will be referred to as the "maximum quiet time".
Data from the local register may be manipulated before being written to the remote register. The local data is first multiplied by the scale factor. The offset is then added to it. If a bit mask is entered, and the remote register type is signed or unsigned integer (16-bit or 32-bit data), the mask will be bit-wise logical AND-ed with the data. The mask is right justified, then AND-ed with the data. The result is then left shifted back to the original position of the mask. In other words, the least significant bits of the original data will be stuffed at the position marked by the mask.
After the scaling and masking, the bit fill will be logically OR-ed into the result, but only if the mask was nonzero and was used. Both mask and fill are entered in hexadecimal. The effect of "fill" is that certain bits will always be set to 1 in the data written to the remote Modbus device.
Multiple local registers may be packed into a single remote register. To accomplish this, define two or more maps in sequence with the same remote destination. If the destination is the same, data types are 16 or 32-bit integer (signed or unsigned), bit masks are nonzero, and the maps are sequential, the results of all qualifying maps will be OR-ed together before being sent to the remote destination.
For the remote register to be written, select the register type, format, number, select a remote TCP device from the list, and enter a unit number if the default unit number for that device should not be used. Data formats are the same as described above for Read Maps. Size is only specified for character strings. Use INT-16 or UINT-16 data format for coils - in this case format only affects local register data conversion.
Modbus protocol treats all holding registers as strictly 16-bit registers. To accommodate 32-bit or longer data, Modbus devices use multiple consecutive "registers" to hold the data. There is no standardization of whether the least significant part of the data comes first or last. Therefore, ValuPoint lets you set that according to whatever the remote Modbus device requires. If the least significant data is found in the first (or lower numbered) register in your Modbus device, then check the box after "With low register first".
The repeat time may determine how often the remote register will be written. If send on delta and maximum quiet time are not checked above, clicking the "at least" button will establish a periodic update time. If send on delta is used and you wish to limit the network traffic in the event changes are frequent, click the "no more than" button and enter the minimum time that should elapse before another write to the remote device. It is valid to select "no more than every 0.0 seconds" if you want all changes to be sent, but no periodic writes.
You have the option of making this Write Map conditional. If an index register number is provided and the Enable box is checked, then this write map will only be executed when the index register (local register) contains the value given. This allows multiple write maps to supply data to the same remote register based on the value of the local index register. It also allows writing to simply be suspended if a single write map supplies data to the remote register. In a more sophisticated scenario, you could potentially suspend writing of the remote device if you know the device is powered down.
Delete will remove the map number shown in the "Map #" box. Insert will insert a new map before the map number shown, and is used for placing maps between existing maps. It is not necessary to use Insert to add maps to the bottom of the list or to define any map presently having zero for a source register or "none" for remote type.
Selecting "none" for remote type effectively deletes the map even though it will still appear in the list until deleted. Unused maps at the end of the list will always show none as the type. If you wish to prevent these from being displayed, reduce the number of maps enabled.
If the local register and remote register are not the same format, then data is converted automatically when written. In the example above, the Write Map is writing register 3, a floating point value, to remote register 2, an unsigned 16-bit value. The data is converted and rounded up, as illustrated below by ModSim acting as our Modbus TCP server here.
The TCP Registers page shows a list of local registers mapped to TCP server devices. The page will show only one device at a time, and may have no entries if there are no maps for that device.
Click the Next Dev or Prev Dev buttons to go to the next or previous TCP device, or simply enter a number in the TCP Device # window and click Update. Registers for that server will now be displayed. In addition to a summary of the map (both read and write maps are shown), the time since last update is displayed. This time should generally be less than the poll time. If the last update time is large, it may mean there is an error preventing the update.
The Error Counts page shows a tabulation, by TCP device, of all errors observed. In the example below, we can see that TCP device #1 is apparently not configured correctly as it is getting as many exception errors as messages sent.
If the counts show some problems, we can look for more detail on the Errors: Read Maps (or Errors: Write Maps) pages. These pages will tell us exactly which Read Map (or Write Map) the problem is occurring on, and what the error is, as illustrated below.
If you see total messages of zero and a "no responses" count greater than zero, it means the ValuPoint was not able to connect to the IP address of the TCP server. Without being able to connect at all, there was never an attempt to send a message, and hence zero total messages while "no responses" continues to increment.
When "no responses" is indicated, the Errors: Read Maps (or Write Maps as applicable) page will show that the response timed out, but this is typically a foregone conclusion when you see the "no responses" count for a TCP device.
When you get any type of connection related problem with a TCP device, the connection status will typically give you some clues.
Connection status codes you may see include:
5 = Connection attempt timed out, unable to establish connection (usually means remote device not connected or not reachable)
104 = Connection reset by peer
111 = Connection refused
113 = Connection aborted
114 = Network is unreachable
115 = Network interface not configured
116 = Connection timed out
118 = Host is unreachable
125 = Address not available
205 = DNS error