Micromod MOD: MODBUS RTU Communications Guide - Manual (Page 11)

MODBUS RTU
COMUNICATIONS GUIDE
7
MODBUS WITH MODCELL MLP AND MOD30ML
STANDARD PROTOCOL
“Standard” MODBUS supports single-register, 16-bit integer values. Modcell and MOD30ML floating-point
data may be converted to this format with ML blocks. For maximum resolution, specify an actual engineering-
unit range in the ML block. If the engineering range is greater than 65535, a scaled range must be selected.
Some standard MODBUS drivers may be capable of reading floating-point data, using a 32-bit two-register
format.
The standard MODBUS driver (MB1) for Fix DMACS does not support the floating point format from Modcell
MLP and MOD30ML.
The standard MODBUS driver for PC-30 and GFW supports Modcell MLP and MOD30ML floating point
signals. A MODBUS I/O Server is also available for GFW, supporting floating-point.
EXTENDED PROTOCOL
Modcell and MOD30ML devices support an extended protocol, in addition to standard MODBUS RTU as a
subset. If the Master is capable of utilizing the extended features, additional data types, diagnostics and event
information may be obtained. The extended protocol also supports the Status Page in the Application Builder
and very fast uploads and downloads. Note that the 2-wire RS-485 module does not support these features.
The Extended MODBUS protocol implemented in Modcell and MOD30ML utilizes a user-defined MODBUS
command, and custom drivers are required in the host to support it. Custom drivers supporting this Extended
MODBUS protocol are available for PC-30, Genesis for Windows and FIX DMACS. The PC-30/GFW driver
supports only Extended MODBUS. The FIX drivers also support standard MODBUS devices, even on the
same network as Extended MODBUS devices.
When using Extended MODBUS, special interface files are created when compiling the instrument database.
These files provide automatic configuration of host poll records, significantly reducing engineering time. A MIF
file is used for FIX software. PC-30 and GFW use a TIF file. With Extended MODBUS, user-friendly alias
names are given to points in the Configured List. In the host software, these alias names are used instead of
register addresses.
MOD 30ML COMMUNICATION CHANNELS
The built-in communication channel on a MOD30ML can be used with front or rear connection, but not
simultaneously. The front connection is RS-232 only, and the rear connection can be either RS-232 or RS-
485. Connecting wires to the rear terminations may prevent communications through the front port, even if the
rear terminations are not being used. Installing a communication module in slots S9-10 or S10 will disable the
built-in channel. A second communication channel can be added by installing a module in slots S7-8 or S8.