5 RG-MHMOD
Data & Register Format
3
Data Representation - Internal and External
3.1 Register Format
All registers are 16 bits. The data within these registers can take on the following forms:
•
Individual bit commands (16 per register).
Example: Register #1 (Modbus Register #2).
•
Individual bit flags (16 per register).
Example: Register #1.
•
A chain of two 8 bit unsigned integers.
•
A 16 bit unsigned integer.
This unsigned integer could represent many different types of data with various scaling rules and
units, which are defined by the DATA TYPE of the register.
3.2 Data Types
Data passed in registers across the Modbus communications link are always in INTERNAL units. The drive
itself may show the information in alternate DISPLAYED units. For Example: drive speeds are always stored
internally as hundredths of a Hz but the drive may display that speed in terms of RPM's using programmed
conversion factors. Table 3 lists examples of the internal units used on the MCH series.
Table 3: MCH Series Internal Units
Type
Unit
Example
SPEED
0.01 Hz
60 Hz = 6000
TIME
0.1 Sec
30.0 Sec = 300
The data type "PID" requires further explanation. The internal range of any data of PID type is 0 to 32736.
Many variables can be controlled in a PID system (pressure, temperature, flow, etc.). To simplify drive
calculation, the drive programmer enters the type and range of the controlled variable (actually it is the
range of the feedback device that we are scaling) and this range is mapped to the internal range (0-32736).
If the feedback device measured 0 to 200 PSI, then 0 PSI is 0 internal units, and 200 PSI is 32736 internal
units. To command a setpoint of 100 PSI the LOCAL PID command (Register #41) can be written with the
value 16368 (32736*100/200). Thus to control PID operations using real world units, the Modbus Master
must have knowledge of the range of the feedback device.
3.3 AC Tech Drive Registers
Registers #0 - #50:
(Modbus Reg #1 - #51) Reserved for configuration and control
Registers #51 - #260
(Modbus Reg #52 - #261) Reserved for drive's programming mode parameters
Programming Mode Parameters are the parameters accessible from the local keypad on the drive. There is a direct
correspondence between the MCH Drive Programming Mode Parameter number and the AC Tech Register number
(and consequently, the Modbus register number):
AC Tech Register # = (MH Programming Parameter #) + 51
Modbus Register # = (MH Programming Parameter #) + 52
The Table 6 entries are based on MCH Drive Software # 213-070 Revision 00 (Parameter Configuration = 148). If a
later revision of software were to change register definitions, drive operation could be seriously affected. Examine
Register #50 (Parameter Configuration Number) for revision. The number displayed at power up on drive display also
identifies it. If it is not 148, writing to any register on the drive MUST NOT BE ATTEMPTED unless your Controller has
been setup to support the new configuration.
