Quest Controls BACnet T-Stat Servisní příručka stáhnout pdf (Strana 218)

218 • Fieldbus Communication

MODBUS Functions

WAGO-I/O-SYSTEM 750

BACnet/IP Controller

4.3.3.1 Function Code FC1 (Read Coils)

This function reads the status of the input and output bits (coils) in a slave de-

vice.

Request

The request specifies the reference number (starting address) and the bit count

to read.

Example: Read output bits 0 to 7.

Byte Field name Example

Byte 0, 1 Transaction identifier 0x0000

Byte 2, 3 protocol identifier 0x0000

Byte 4, 5 length field 0x0006

Byte 6 unit identifier 0x01 not used

Byte 7 MODBUS function code 0x01

Byte 8, 9 reference number 0x0000

Byte 10, 11 Bit count 0x0008

Response

The current values of the response bits are packed in the data field. A binary 1

corresponds to the ON status and a 0 to the OFF status. The lowest value bit of

the first data byte contains the first bit of the request. The others follow in as-

cending order. If the number of inputs is not a multiple of 8, the remaining bits

of the last data byte are filled with zeroes (truncated).

Byte Field name Example

.....

Byte 7 MODBUS function code 0x01

Byte 8 Byte count 0x01

Byte 9 Bit values 0x12

The status of the inputs 7 to 0 is shown as byte value 0x12 or binary 0001

0010. Input 7 is the bit having the highest significance of this byte and input 0

the lowest value. The assignment is thus made from 7 to 0 with OFF-OFF-

OFF-ON-OFF-OFF-ON-OFF.

Bit: 0 0 0 1 0 0 1 0

Coil: 7 6 5 4 3 2 1 0

Exception

Byte Field name Example

.....

Byte 7 MODBUS function code 0x81

Byte 8 Exception code 0x01 or 0x02

1 2 ... 213 214 215 216 217 218 219 220 221 222 223 ... 359 360

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Quest Controls BACnet T-Stat Servisní příručka Strana 218