Camille Bauer SINEAX V604s Manuel d'utilisation - Page 8

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6. Modbus interface

6.1 EIA-RS-485 Standard

The EIA-RS-485 standard deﬁ nes the physical layer of the

Modbus interface.

Coding

The data is transmitted in serial form via the 2-wire bus. The

information is coded as a difference signal in the NRZ code.

Positive polarity signals a logic 1, negative polarity signals

the logic 0.

Connections

A shielded, twisted, 2-conductor cable should be used as

a bus cable. Shielding serves improved electromagnetic

compatibility (EMC). Depending on the source of informa-

tion, the description of Conductor A and B is contradictory.

The potential difference of all bus participants may not exceed

± 7V. Therefore, the use of a shield or a third conductor (ref

line) is recommended to create potential equalisation.

+ line

– line

ref line

Topology

Both ends of the bus cable

must be equipped with a line

terminator. Supplementing the

line termination resistance RT

of the EIA-RS-485 standard an

additional resistance RU (pul-

lup) must be wired against the

supply voltage and a resistance

RD (pulldown) against the refer-

ence potential. These two resistances ensure a deﬁ ned idle

potential on the line when none of the participants is sending.

Slave

Master

Slave

Master

Slave

+5V

390

RS-485 A/P +

220

RS-485 B/N –

390

ref line

0 V

Slave

Operating instructions V604s Be

System requirements

Cable:

Twisted, 2-wire line, wave resistance 100

to 130 Ω, min. 0.22mm

Line length:

Maximum 1'200m depending on the

transmission rate

Participants:

Maximum 32 per segment

Rate:

9'600, 14'400, 19'200, 38'400, 56'000,

57'600, 115'200 Baud

Mode:

11 bit format - 2 stop bit without parity

or 1 stop bit with even/uneven parity

6.2 Coding and addressing

Addressing

In the telegram, all data addresses refer to zero. The ﬁ rst data

element is always addressed via the 0 address. For example,

the coil which is known as "Coil 1" in the device, is addressed

as "Coil 0" in the telegram. Coil 127 is addressed as 0x007E.

Holding register 40001 is addressed as Register 0 in the

telegram. The function code of the telegram already states

that a "holding register" is concerned. Consequently, the

reference to "4XXXX" is implicit.

Holding register 40108 is addressed as 0x006B (107 decimal)

Serialisation

The speciﬁ cation deﬁ nes the telegrams as byte sequences.

The respective physical layer (RS485, Ethernet) is responsible

for the correct serialisation of the bytes (MSB or LSB First).

RS485 (UART, COM) transmits the "Least Signiﬁ cant Bit"

ﬁ rst (LSB First) and adds the synchronisation and backup

bits (start bit, parity bit and stop bit).

Start

Bits

Bits are represented within a byte in a conventional manner

with the MSB (Bit 7) leftmost and the LSB (Bit 0) rightmost

(0101'1010 = 0x5A = 90). An example for the inquiry of Coils

20 to 40 of Slaves 17.

Byte

Inquiry

Slave address

Function code

Start address

19 = Coil 20

Number

20...40 = 21

The start address in the inquiry plus the bit position in response

byte 0 corresponds to the coil address. Commenced bytes

are completed with zeros. Coil 27...20 = 0xCD = 11001101b

→ Coil20 = ON, Coil21 = OFF, Coil22 = ON, etc.

(24AWG)

Par Stop

Response

0x11

Slave address

0x11

0x01

Function code

0x01

0x00

Byte count

0x03

0x13

Byte 0

0xCD

0x00

Byte 1

0x6B

0x15

Byte 2

0x01

Camille Bauer