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28 LD Protocol

5 LD Protocol

5.1 Communication Parameters

5.2 Command format

5.2.1 Telegram structure

Master sends

Slave answers

Data format

Baudrate 19.200, 8 data bits, 1 stop bit, no parity

ENQ LEN ADR CmdH CmdL DATA (n bytes) CRC

012 345 5+n

STX LEN StwH StwL CmdH CmdL DATA (n bytes) CRC

0123456 6+n

Command Meaning

ENQ 0x05 Start of master request

STX 0x02 Start of slave response

LEN

Number of

telegram bytes

without ENQ(STX)/LEN, however with CRC

max. 253, so the total slave telegram length is max.

255

ADR Slave address

Slave address = 1: non-addressed bus. Address byte

is ignored.

Stw H/L Status word Info from slave to master (5.3)

Cmd H/L Command

Bit 15 ... 13: Command-specifier Read/Write etc. (see

table “Cmd H/L: Command: Command-specifier”)

Bit 12: free

Bit 11 ... 0: Command (5.3)

DATA

Data belonging

to master

request

(Slave reply to

write command

is sent without

data)

0 <= n <= 248

If I/O module (7-byte additional header) is used, then

limit maximum data length to 241.

CRC Checksum

Calculate CRC for all bytes (except CRC byte)

Polynomial: 0x98,

Name: DOWCRC,

Maxim/Dallas, X^8+X^5+X^4+1

Info:

CRC calculation see document "CRC_calculation.c"

(C souce code)

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 55 56

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