Modbus TCP Client Write Map Edit

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Modbus TCP Client Write Maps are where you configure this device to write to Modbus registers in other Modbus TCP devices, taking data to be written from local data objects. This page is where you enter the various parameters to make that happen.

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Map Number – Used as a reference in the map list for ordering the maps. Polling is done in round robin fashion in the order of map number.

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Source Object – Specifies the local object number that contains the data that should be sent by this write map.

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Scale – Provides a scale factor if non-zero (has the effect of being 1 if zero). Data to be written is retrieved from the local object and then multiplied by this scale factor before being sent to the remote Modbus device. Applies to numeric values and numeric local objects only.

Offset – Provides an offset to work in conjunction with scale factor. This value is added to the value retrieved from the local object (after being multiplied by scale) before being sent to the remote Modbus device. Applies to numeric values and numeric local objects only.

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Mask – A bit mask given as a 8-digit hexadecimal value, if non-zero. The mask operation skipped if mask value is zero, or register format is not Int (integer). When the data of interest is a single bit, or bit field less than the full register width, the Mask is used. The process used in a read operation is reversed here. First, the mask is right justified so that the least significant “1” bit is in the least significant data position. That mask is then logically ANDed with the data found in the local object. The result is then left justified back into the position originally indicated by the mask. This value is now ready to be written to the Modbus register, pending any additional operation such as the Fill mask.

Fill – An additional bit mask given as a 4-digit or 8-digit hexadecimal value. This mask is logically ORed with the result of the Mask operation before the final result is written to the Modbus register. The Fill mask has the effect of making sure certain bits in the register are always set.

NOTE: The order of operation is as follows, operating on data retrieved from the local object: (1) apply scale if nonzero; (2) apply offset; (3) apply mask if applicable; (4) apply fill if applicable; (5) write to Modbus register.

The appearance of the following line will vary depending on your user settings. You have the option of displaying Modbus registers as raw address (0-indexed), register number (1-indexed), or Modicon format (e.g. 40001 style).

The following variations all refer to exactly the same register:

Modbus write map edit 5a.jpg Modbus write map edit 5a-addr.jpg Modbus write map edit 5a-modicon.jpg

Click the Check to validate a Modicon number.

The options available on this line will vary depending on selections made. The following are a few examples.

Modbus write map edit 5a.jpg Modbus write map edit 5b.jpg Modbus write map edit 5c.jpg Modbus write map edit 5d.jpg Modbus write map edit 5e.jpg

Register Type – Modbus register types available are listed in the following table. These labels are illustrated here as recognized in XML or CSV files, but are further annotated on the web page.

Label Modbus Register type
“none” No register defined
“Coil” Coil
“Disc” Discrete Input
“Input” Input Register
“Hold” Holding Register

Register Number or Address – Enter the number (starting at 1) or raw address (starting at 0) as applicable. Do NOT enter 40001 for holding register 1 if you have not selected Modicon as the display format in your User Settings.

Modicon Register – Enter numbers like 40001 for the first holding register if you have selected Modicon representation in your User Settings.

Register Format – Select the format of the data contained in the Modbus register(s). This is not used by the protocol, but is used by the gateway to interpret what the raw increments of 1 or 16 bits should mean. Select format from the following table.

Format Label Format description
“None” No format defined
“Bit” Single bit, used ONLY for Register Type Coil or Disc
“Int” Integer (size and whether signed are defined by labels below)
“Real” Floating point (single or double precision)
“Char” Character string with 2 ASCII characters per register
“Mod10” Mod10 format, can be 2, 3, or 4-register, specific to Schneider Electric meters

Register Size – Register size refers to the number of consecutive input or holding registers that should be written for a value greater than 16 bits. A 16-bit value would have size of 1, a 32-bit value would have size of 2, and a 64-bit value would have size of 4. Single precision Real (32-bit IEEE 754 floating point) would be size 2, and double precision Real (64-bit IEEE 754 floating point) would be size 4. If format is Mod10, then valid sizes are 2, 3, or 4 – check manufacturer’s documentation if Mod10 is noted. Register “size” for a character string will be character count divided by 2 (plus 1 of string length is an odd number). Register Size is not used for Coil or Disc types.

Device – (REQUIRED if TCP) – Device number from the TCP Device list that should be accessed for this write attempt on a TCP network.

Unit – (TCP only) – Unit number to be included in the TCP request, will default to 1 if not given or is set to zero.

UseFC56 – Enter “Y” to force single register writes to use Modbus function 5 to write a single coil, or function 6 to write a single holding register. Function codes will default to “write multiple” function codes 15 and 16 instead of 5 and 6 respectively if “N” is entered or this column is omitted.

Unsigned – Indicate “Y” if unsigned, or “N” if signed. Defaults to signed integer. Has no effect on RegFormat other than Int.

LittleEnd – Used when RegSize is greater than 1 to indicate what order the registers should be interpreted in. Enter “Y” to indicate that the lowest numbered register contains the least significant portion of data. Enter “N” or omit to indicate that the lowest numbered register contains the most significant portion of data. Although Modbus protocol itself is not inherently “Little Endian”, many devices operate that way due to Intel processors being inherently Little Endian. Modbus protocol does not stipulate what the register order should be when multiple registers are treated as a single data entity. Therefore, the user is required to pay attention to this.

SendPeriodic – Set to “N” to disable, or “Y” to enable periodic writing of the Modbus register at the poll rate given by PollTime.

PollTime – Poll time in seconds, can be fractional. This poll time is not guaranteed to be met. Polling is done in round-robin fashion. In a very busy system, more than this time may expire before the next poll. If less than this time has expired, then the system will wait this amount of time until polling again. The sets the rate at which the remote Modbus register will bewritten, provided “SendPeriodic” has been enabled. This poll time will be disregarded if SendPeriodic is not enabled.

SendMaxQuiet – Set to “N” to disable or “Y” to enable the MaxQuietTime feature. If disabled, the MaxQuietTime will be disregarded.

MaxQuietTime – “Max Quiet” time in seconds, can be fractional. If the Modbus register has not been written either as a result of poll timing or value changing by delta within this time period, then write request will be made anyway. This specifies the maximum amount of time that should expire without any write to the Modbus register for any reason.

SendOnDelta – Set to “N” to disable, or “Y” to enable the “send on delta” feature where Modbus writes are made based on changes in the local object value (see delta below).

Delta – Specifies the margin by which the local object value should change before sending another Modbus write request to the remote Modbus device. Once the changed value has been sent, the new local value is retained for future comparison in determining subsequent additional change. The delta value is disregarded if SendOnDelta is not enabled. Note that a delta of zero is treated as a special case: Any update to the local object by any process will result in a new Modbus write request.

MinQuietTime – Time in seconds, can be fractional. This specifies the mínimum amount of time that should elapse between sending of write requests for this write map. The minimum quiet time has the effect of throttling network traffic.

IndexObj – Optional, allows for selectively enabling this write operation. If an index object (local object number) is given, and its value matches the IndexVal value, then this write operation will take place. If an IndexObj is given but the local object’s value does not match the IndexVal, then this write operation will be skipped.

IndexVal – Optional, used in conjunction with IndexObj (see note above).