3300 ACM Installation and Operation Manual Power Measurement Ltd.
5-6 Communications
The cable connecting the Bridge/Mux to the converter is a 9-
pin male to 25-pin male serial cable. Refer to Section 2.6 for
required RS-485 network connections.
NOTE
The Modbus protocol is extremely sensitive
to signal reflections which may be caused
due to an unterminated bus. Refer to Sec-
tion 2.6 for information on bus termina-
tion.
Setting Communication Parameters
When using Modbus communications, the range of possible
UNIT ID designations for the 3300 ACM is limited to between
1 and 247.
The COM MODE parameter must be set to RS-485.
A setup parameter named PROTOCOL has been provided to
select between Power Measurement and Modbus communica-
tions standards. If the 3300 ACM is to be connected to a
Modbus communications network, the PROTOCOL param-
eter should be set to MODBUS.
If PROTOCOL is set to Modbus, the next parameter that
appears is REGISTER SIZE. This parameter determines
whether a 16-bit or an extended 32-bit register is passed in
communications for each function. The default setting is 16
BIT.
NOTE
TRAN models (without display/keypad
module) that are intended for use with
Modbus communications should be or-
dered with the MOD option. For this op-
tion, the factory presets the PROTOCOL
parameter to MODBUS.
Communications Protocol
Communications occurs from the Controller via the Modbus
Plus network (using MSTR block), across the BM85 to the
Modbus, and on to the 3300 ACM(s) via the RS-232C / RS-
485 converter.
All communications between the BM85 and 3300 ACM(s)
conform to a master/slave scheme with the BM85 as the
master and the 3300 ACM(s) as slave(s).
Message Packets Supported
All registers within the 3300 ACM are accessible as PLC 4xxxx
holding registers. The following Modbus commands are
supported:
PRESET MULTIPLE REGISTERS (10H)
Allows the Modicon Controller to define all the user-program-
mable setup parameters in the 3300 ACM. Registers are also
provided to allow the Controller to clear the KW Hour, KVA
Hour, or KVAR Hour counters of the 3300 ACM.
READ HOLDING REGISTERS (03H)
Allows the Controller to read 16-bit or 32-bit real-time
measured data or setup parameters from the 3300 ACM.
For a detailed specification describing the 3300 ACM Modbus
communications protocol, refer to the Power Measurement
Ltd. document:
3300 ACM / Modicon Modbus
Serial Communications Protocol
This document is available upon request from Power Mea-
surement or your local Power Measurement representative.
Courtesy of NationalSwitchgear.com
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