Entec LBS-751, Quick Start Manual

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Quick Start Guide APPENDIX 2
LBS-751 Control
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03h Read Holding Register Command
Use function code 03h to read holding register values containing “bit words” or analog data.
To determine data address, add 125 (decimal) to Setting location in the Modbus User-
Defined Assignment Map above.
Example 1:
Phase-A Current Demand (IAD) is available using the 03h function to
read address 133 (Mod_08 Setting lo 125 = 133).
Example 2:
AC Power Fail is available using the 03h function to read address 169 as
noted in the Status Bit table above. AC Power Fail can be determined from the state of bit 1
using polarity convention where Logic 1 = True Condition. The Scada master will typically
collect 16 bits (2 Bytes) from address 169. The desired bit location is determined counting
from Right to Left starting with Bit-0. The second bit counting from the right is Bit-1
representing AC Power Fail. (1 = AC Power Fail True, 0 = False).
Bytes Field
03h Read request format from Master
1 byte Slave Address
1 byte Function Code (03h)
2 bytes Starting register address
2 bytes Number of registers read ( n )
2 bytes CRC‐16
03h Successful response format from Slave
1 byte Slave Address
1 byte Function Code (03h)
1 byte Bytes of data ( n )
n bytes Data (2‐250 Bytes)
2 bytes CRC‐16
03h command will collect all data from the LBS-751 User-Defined Assignment Map with a
single poll (125 registers). In many systems this method will collect Binary and Analog
points with a single poll allowing more devices to communicate with the Scada master.
Large Scada systems utilize 03h to collect binary words versus using 02h to retrieve single
binary bits.