Network Description
NODE: a network device, typically a PLC, PC, instrument or transducer.
MASTER: a NODE which controls the protocol of a MASTER-SLAVE network.
SLAVE: a node which responds to specific requests from a MASTER node.
ADDRESS: a node’s unique network identifier in the range of 1-247.
PACKET: a string of bytes transmitted sequentially across a network.
FIELD: a byte or sequence of bytes within a packet of data.
CYCLIC REDUNDANCY CHECK (CRC): an packet’s error check value.
PROTOCOL: a set of rules or a procedure.
A node’s unique address allows the master to selectively request data from a specific node. The master node, under the control of a program, requests measurements from one of its slave nodes using an established protocol. (Modicon’s Modbus)
For more information on the Modbus protocol and to download relevant literature go to http://www.modbus.org.
Format
Baud rates supported (not all units): 57600, 38400, 19200, 9600, 4800 and 2400.
Bit rates (1/baud rate) are 17.36us, 26us, 52us, 104us, 208us and 417us respectively.
The format, as specified by Modicon, is a constant 11 bits per byte transmitted and received, consisting of 1 start bit, 8 data bits, 1 parity bit and 1 stop bit.
If NO parity bit is used there should be an extra stop bit inserted in its place (i.e. 1 start bit, 8 data bits and 2 stop bits).
N.B. In more recent units a format of 10 bits is supported, i.e. 1 start bit, 8 data bits, NO parity bit and 1 stop bit.