snap ethernet-based i/o units protocols and programming guide

CHAPTER 5
Chapter 5
Advanced Modbus Programming
Introduction
Opto 22 form #1460, the SNAP Ethernet-based I/O Units User’s Guide, shows basic
communication with a Modbus master using Modbus/TCP. However, if you need to read and
write data in other areas of the brain’s memory map not covered by the Modbus memory map,
you can do so with a SNAP-UP1-ADS or SNAP-B3000-ENET brain. This chapter shows you how.
This chapter assumes that you already have an understanding of Modbus/TCP programming and
communications. Use this chapter in conjunction with the Open Modbus/TCP Specification and
the Modicon Modbus Protocol Reference Guide, both available from Modicon, Inc., Industrial
Automation Systems. Information is also available at www.modbus.org/modbus tcp.htm.
NOTE: Digital-only brains do not support advanced Modbus programming. Use an
analog/digital/serial brain instead.
Accessing Other Data in the Brain’s Memory Map
If you need to read and write data in areas of the brain’s memory map not included in the Modbus
memory map, you can do so with a SNAP-UP1-ADS or SNAP-B3000-ENET brain. This section
shows you how.
All of the brain’s regular memory map addresses are available for reading and writing, with the
exception of PID module and data logging addresses.
As explained in the Modbus appendix in the SNAP Ethernet-based I/O Units User’s Guide, SNAP
Ethernet-based brains use a Modbus packet inside TCP/IP. The Modbus checksum is not used;
instead, the Ethernet TCP/IP link layer checksum guarantees data. The size of the packet is
limited to 256 bytes.
The Modbus master needs to support Class 0 commands and the following function codes:
03
06
10
Read holding registers
Preset single register
Preset multiple registers
SNAP Ethernet-Based I/O Units Protocols and Programming Guide 99