Owners manual

6: Troubleshooting and Technical Support
Modbus Protocol User Guide 24
Table 6-1. Baud Rate
Baud
Rate
Byte/Sec Bit Time
(msec)
Byte Time
(msec)
256 Byte Time
(msec)
(in sec)
300 30 3.333333 33.333333 8533.333333 8.53
600 60 1.666667 16.666667 4266.666667 4.27
1200 120 0.833333 8.333333 2133.333333 2.13
2400 240 0.416667 4.166667 1066.666667 1.07
4800 480 0.208333 2.083333 533.333333 0.53
9600 960 0.104167 1.041667 266.666667 0.27
19200 1920 0.052083 0.520833 133.333333 0.13
38400 3840 0.026042 0.260417 66.666667 0.07
57600 5760 0.017361 0.173611 44.444444 0.04
115200 11520 0.008681 0.086806 22.222222 0.02
The overall time it takes to poll is the combined sum of these delays:
a. Delay for master/client to recognize need for poll.
b. Delay to issue and get the poll onto the Ethernet.
c. Delay for the poll to cross Ethernet and arrive error-free at the IAP Device Server
device (may include retries and contention).
d. Delay for IAP Device Server to process and queue Modbus/RTU poll.
e. Delay for the serial link to be free (remember other master/clients may be actively
polling).
f. Physical delay to shift poll bit-by-bit across the serial link.
g. Delay in the device to recognize, process, and start reply.
h. Physical delay to shift response bit-by-bit across the serial link.
i. Delay for IAP Device Server to process and queue Modbus/TCP response.
j. Delay for the response to cross Ethernet and arrive error-free at the master/client
(may include retries and contention).
k. Delay for master/client to recognize need for poll.
Delays a and k are defined by your OPC or DDE driver. For example, a driver
that runs only once each 55 msec (using the old DOS timer slice) can have a
variable delay here of between 0 to 110 msec.
Delays c and i are defined by the complexity and load of your TCP/IP network.
For example, if you are going through radio or satellite links, these delays
routinely amount to 1000 msec (1 sec) or more per poll and another 1000 msec
for a response.