3300 ACM Installation and Operation Manual Power Measurement Ltd.
Appendix G: Serial Communications Protocol G-1
APPENDIX G
SERIAL COMMUNICATIONS PROTOCOL
NOTE
The information contained in this docu-
ment is believed to be accurate at the time
of its publication; however, Power Mea-
surement Ltd. assumes no responsibility
for any errors which may appear here and
reserves the right to make changes without
notice.
1 INTRODUCTION
This document details the Power Measurement serial commu-
nications protocol used to pass commands, information and
data into and out of the model 3300 ACM Power Meter.
Provided is all the information necessary for a 3rd party OEM
to develop in-house software to communicate with a 3300
ACM.
Only the basic Power Measurement protocol is described here.
For detailed information regarding the 3300 ACM / Modbus
communications protocol, contact Power Measurement or
your local representative.
1.1 PURPOSE
The purpose of the communications protocol is to allow
information and data to be efficiently transferred between a
central data collection station (Master Station) and a 3300
ACM Power Meter. This includes:
1) Allowing configuration and interrogation of all 3300
ACM power meter setup parameters from the Master
station.
2) Allowing interrogation of all data measured by a 3300
ACM power meter.
1.2 REVISIONS
May 24, 1991 Initial release.
October 1, 1993 Decimal accuracy and high resolution
request packets added.
2 DETAILED DESCRIPTION
2.1 PROTOCOL GROUND RULES
The following rules define the protocol for information
transfer between the RS-485 loop controller and other
components of the RS-485 serial communications loop.
1) All communications on the RS-485 loop conforms to a
MASTER/SLAVE scheme. In this scheme, information
and data is transferred between a single MASTER loop
controller and up to 32 SLAVE monitoring devices.
2) The MASTER will initiate and control all information
transfer on the RS-485 communications loop.
3) Under no circumstances will a SLAVE device initiate a
communications sequence.
4) All communication activity on the RS-485 loop occurs in
the form of “PACKETS”, a packet being simply a serial
string of 8 bit bytes. The maximum number of bytes
contained within one packet is 255.
The bytes that comprise a packet consist of standard
asynchronous serial data transmitted with 8-bits per
data byte, no parity and one stop bit. The serial data
streams are generated using equipment similar to that
used for RS-232C.
5) All transmissions on the RS-485 loop can be divided into
two types of packet activity:
i) Master to Slave transmissions
ii) Slave to Master transmissions
These two packet types are distinguished via a
“sync” byte that is transmitted as the first byte of
every packet.
For Master to Slave transmissions,
sync = 00010100B = 14H
For Slave to Master transmissions,
sync = 00100111B = 27H
6) In the case where the Master or any Slave device receives
a packet that contains an unknown command, the
packet shall be ignored and no further response will be
made by the receiving unit.
Courtesy of NationalSwitchgear.com
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