AutomationDirect DURAPULSE GS3-2010, User Manual

Page: 224 / 316

Chapter 5:
DURA
PULSE MODBUS Communications
5–74
Comm Delay – Optimizing Communications
Optimizing Communications to GS Drives
If you are only communicating to one or two drives and reading or writing only a
few parameters, the communication speed will most likely be sufficient for your
application.
However, in the case that the communication speed (reaction time from reading
or writing an event to a given drive) is too slow, you may need to take a more
detailed look at how your code is designed to communicate to the GS Drives in
your application.
To properly design the system, it is necessary to understand all of the propagation
delays that are incurred when triggering the event to send a Modbus message to
the point of receiving the data or status of the reply into the PLC or Modbus
master.
To determine the time necessary to transmit a message from the Master to the
Slave and vice versa, we must first determine the “Bit Time” and the “Character
Time”. This is calculated by using the following formulas:
• Bit Time:
The value one divided by the baud rate. A baud rate of 19,200 equals a bit rate of
0.0000528 (1/19200) or 52 µs (microseconds).
• Character Time:
Bit Time multiplied by the number of bits. With Modbus this is typically 10–12
bits per character [1 start bit (fixed), 1 or 2 stop bits (usually configurable), 0 or 1
parity bit (Odd & Even = 1 bit; None = 0), & 8 data bits]. For a setting of Odd
parity and 1 Stop bit, this would be 11 bits. So at 19200, Odd parity and 1 stop
bit, a character time would be 0.000573 or 573 µs (0.0000528 · 11).
Now that we know the byte time, we can multiply that time by the number of
characters in each message.
In most cases, optimizing communications to GS Drives MAY NOT BE NECESSARY.
DURA
PULSE AC Drive User Manual 1st Ed. Rev. D 05/2013