Super Systems AC20 RS485 Modbus - Manual (Page 4)

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TITLE: MODBUS/J-BUS Protocol for SSi’s- AC20 ENG. 824E REV. 1
INTRODUCTION
This half duplex protocol accepts one master and one or more slaves.
The physical interface should be of the RS-485 type.
A single multidrop link can take up to 128 devices having the same “High input impedance”
as the transceiver used..
The computer should be programmed to serve as a master controlling which slave has
access to the link. All other slaves are in waiting state. Each slave has a unique address
ranging from 1 to 255.
Address “0” is a broadcast one. When the master sends a message with address 0, all
slaves receive it and no one replies.
TRANSMISSION FORMAT
The protocol uses the RTU (Remote terminal unit) mode of transmission.
RTU is a binary method with byte format composed as follows:
1 start bit, 8 data bit, 1 parity bit (optional), 1 stop bit.
The communication speed is selectable among 600, 1200, 2400, 4800, 9600 and 19200
baud.
COMMUNICATION PROCEDURE
The communication can be initiated only by the master unit; the slave units can transmit
only after a query has been received from the master.
The general format for the transmission from master to slave is the following:
RANGE BYTE
Slave address 1
Function code 1
Data n
Error check (CRC-16) (low byte) 1
Error check (CRC-16) (high byte) 1
The slave detects the start of a query frame when the delay time between two characters
is greater than 3.5 T.U. (Time Unit = Time necessary to transmit one character).
ERROR CHECK(CRC-16 Cyclical Redundancy Check)
The CRC-16 value is calculated by the transmitting device. This value is appended to the
message. The receiving device recalculates a CRC-16 and compares the calculated value
to the received value. The two values must be equal.