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

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.