User Manual

IB-23C650M

MODBUS PROTOCOL

Communications Speed

The controller supports baud rates of 150, 300, 1200, 2400, 4800, 9600, 19.2K,

and 38.4K baud. Messages are not buffered. A new message (including

broadcast type) will not be accepted until an existing message has been

processed and responded to if necessary.

Message Response Time

The controller typically takes between 10 and 30 milliseconds to respond to a

message requesting 10 registers. This time is from the end of the request

message to the start of the response message.

The total amount of time a message takes to get back and forth between the

host and the slave depends on the message size, baud rate and the slave's

time to process the message. The following information is for a request

message to read 10 registers:

• request message size is 8 characters + 3.5 idle time characters (11.5).

• response message size is 25 characters + 3.5 idle time characters (28.5).

• for this example, a character is 11 bits (1 start, 8 data, 1 parity, 1 stop).

• Therefore, the total typical time using a controller response time of 30

milliseconds is shown below for the various baud rates:

Baud Rate

Request Time

(mSec)

Response

Time (mSec)

Total Typical

Time (mSec)

150 623 2090 2743

300 312 1045 1387

1200 78 261 369

2400 39 131 190

4800 19 65 114

9600 10 33 73

19200 5 16 51

38400 2 8 40