3
RG-SFMOD
Introduction
2
Introduction
This document defines the specifics required for Modbus serial communication with a Lenze-AC Tech
standard SCF Series drives for control, status monitoring, and programming parameters. A familiarity
with normal drive capabilities and operations is assumed. If this is not the case, refer to the SCF Series
Installation and Operation manual (SF01) for more information.
2.1 Modbus Details
A. AC Tech Drives running the Modbus communication protocol use the RTU (Remote Terminal Unit)
transmission mode and are slaves only. Therefore, the device communicating with the drives must be
a Modbus Master. The baud rate is 9600, no parity (two stop bits). There are provisions for Odd parity
1 stop bit and Even parity 1 stop bit as well. The bit sequence is:
DATA
Start bit
1
2
3
4
5
6
7
8
Stop bit
Stop bit
B. At this time the AC Tech drives do not support the broadcast function of the protocol.
C. IMPORTANT NOTE: Modbus 3X and 4X Registers are numbered starting at 1. However, when
transmitted to a slave over the serial link, the actual address transmitted is one less. This is because
the addresses are numbered starting from 0. AC Tech register numbers are also numbered starting
from 0. Therefore, AC Tech register numbers always correspond exactly with the address transmitted.
As a result, MODBUS REGISTER NUMBERS ARE ALWAYS ONE GREATER THAN AC TECH REGISTER
NUMBERS. WHENEVER THE WORDS “REGISTER #xx” APPEAR, IT SHOULD BE ASSUMED THAT THEY
MEAN “AC TECH REGISTER xx” and the Modbus Register number will be one larger. In some instances
we may show both for clarity. For example: “Register #24 (Modbus Register #25) . . .”
D. The function codes supported by AC Tech drives are:
03 Read Holding Registers (4X references). In general we can read only one register at a time. However,
there are a few limited exceptions.
Exception One:
Register #24 (Modbus Register #25) Drive Status, can also be read as a group of 6 words.
Exception Two:
AC Tech uses a method of reading a group of related registers that may not be consecutive within
the drive memory map. When this is done for the registers below, the response from the drive will
be for the number of words requested but will not be with consecutive registers.
Register #100 (Modbus Register #101), Fault history, should be read as a group of 4 words.
Register #101 (Modbus Register #102), Software version, should be read as a group of 4 words.
04 Read Input Registers (3X references). As with function 03, we read one register at a time except
where noted.
