Nexen RSD200 964533 - Manual (Page 3)

3 FORM NO. L-21193-A-0904
INTRODUCTION
NOTE: This manual assumes the reader is already familiar with the MODBUS RTU over Serial Line protocol. If not,
then the reader must read the following documents MODBUS Application Protocol Specification and MODBUS over
Serial Line Specification & Implementation Guide found at http://www.modbus.org. In addition, the reader is as-
sumed to have read the RSD200 User Manual.
Modbus RTU is a master/slave protocol that only allows the RSD200 to send information after it has been requested
from the master device, typically a HMI. As a result, messages can arrive at the RSD200 at anytime and interrupt the
processing of the control algorithm. After receiving the message, the RSD200 will wait until it is convenient to send a
reply. Therefore, the master device must have a response time-out that is greater than 200 milliseconds. Generally
speaking baud rates should be high as practical to minimize message transmission time and communication with the
RSD200 should be kept to a minimum to minimize interruptions of the control algorithm.
MODBUS RSD200 dancer controllers feature a RS232 line driver/receiver on board and will require an external RS232
to RS485 converter to be able to communicate on a Modbus network that utilizes the MODBUS RTU over Serial Line
protocol and a full duplex RS485 physical layer. Communication port values are preset to 19200 baud, 1 start bit, 8
data bits, no parity, 2 stop bits and device address 23. Only function codes 03, Read Holding Registers, 06, Write Single
Register, and 16, Write Multiple Registers, and their respective error codes 131, Read Holding Registers, 134, Write
Single Register, and 144, Write Multiple Registers, have been implemented. Following the successful transmission of a
message from the MODBUS Master the RSD200 will send a response appropriate for the function code per the
MODBUS specification.
Table 1 is a summary of the registers used by the RSD200. These 16 bit words are sent as integers with the most
significant byte first and are addressed using base 0 protocol addresses. Some Programmable Logic Controllers will use
base 1 protocol addresses. The registers are grouped by their function of Setup, Calibration, Tuning, and Control
variables and are read with function code 03, and written to using function code 06 or 16.
SETUP REGISTERS
Setup registers are read or written to during the installa-
tion and start up of a RSD200 system and seldom change
afterwards.
ADDRESS
10: Restore Default Values will restore default values
to all variables when a 1 is written to this register.
This is a write only register reading from it will only
return a zero.
11: Load Communication Parameters will load
previously assigned values for Stop Bits, Device
Address, Baud Rate, and Parity into the RSD200
and begin communicating with them when a 1 is
written to this register. This is a write only register
reading from it will only return a zero.
Note: Changes to registers 12 – 15 will only take effect
after a write to register 11 or the RSD200’s power is
cycled off and on. Reading registers 12 – 15 will indi-
cate the communication values currently in use and
not the pending changes. Rotating the RSD200’s ro-
tary switch to position “F” and cycling power off and
on will load the default values for registers 12 – 15. If
registers 12 – 15 are changed, then rotate the rotary
switch to any non-”F” position or the default values will
be used after the RSD200 is turned on.
12: Stop Bits is the number of stop bits sent with each
Modbus message byte. It has values of 1 or 2 and
is read and written to as an integer. The default
value is 2.
13: Device Address is the RSD200’s address on the
Modbus network. It has a range of 1 to 247 and is
read and written to as an integer. The default value
is 23.
14: Baud Rate determines the speed of the message
transmissions. The baud rates supported are 9600,
19200, and 38400. Values are read and written as
integers. The default baud rate is 19200.
15: Parity can be even, odd, or none. Set this register
to 0 for none, 1 for odd, and 2 for even. Values are
read and written as integers. The default parity is 0.
Note: No Parity requires 2 Stop Bits and even or odd
Parity requires 1 Stop Bit per the Modbus standard. If
the no parity and one Stop Bit combination is selected,
then the RSD200 will communicate using a non-Modbus
standard 10 bit packet format. Other combinations of
these two parameters are not supported.