Prosoft-technology MVI56E-MCMR Manual de usuario descargar pdf (Pagina 183)

MVI56E-MCMR ♦ ControlLogix Platform Reference

Modbus Communication Module with Reduced Data Block User Manual

ProSoft Technology, Inc. Page 183 of 225

May 13, 2014

6.6.4 Read Holding Registers (Function Code 03)

Query

Read Holding Registers (03) allows the user to obtain the binary contents of

holding registers 4xxxx in the addressed Slave. The registers can store the

numerical values of associated timers and counters which can be driven to

external devices. The addressing allows up to 125 registers to obtained at each

request; however, the specific Slave device may have restriction that lower this

maximum quantity. The registers are numbered form zero (40001 = zero, 40002

= one, and so on). The broadcast mode is not allowed.

The example below reads registers 40108 through 40110 from Slave 584

number 11.

Adr

Func

Data Start Reg Hi

Data Start Reg Lo

Data #of Regs Hi

Data #of Regs Lo

Error Check Field

CRC

Response

The addressed Slave responds with its address and the function code, followed

by the information field. The information field contains 1 byte describing the

quantity of data bytes to be returned. The contents of the registers requested

(DATA) are two bytes each, with the binary content right justified within each pair

of characters. The first byte includes the high order bits and the second, the low

order bits.

Because the Slave interface device is normally serviced at the end of the

controller's scan, the data will reflect the register content at the end of the scan.

Some Slaves will limit the quantity of register content provided each scan; thus

for large register quantities, multiple transmissions will be made using register

content from sequential scans.

In the example below, the registers 40108 to 40110 have the decimal contents

555, 0, and 100 respectively.

Adr

Func

ByteCnt

Hi Data

Lo Data

Hi Data

Lo Data

Hi Data

Lo Data

Error Check Field

CRC

1 2 ... 178 179 180 181 182 183 184 185 186 187 188 ... 224 225

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