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Modbus AB SLC500 with PROSOFT slow
Improving data transfer speed
By Mário R Domenici on 19 July, 2002 - 10:33 am

Dear Colleagues:

We have an application running with PLC SLC500 S/03 with PROSOFT MCM 3150 card (working as Modbus
Master RTU module - RS422, 19200Kbaud) it controls an Industrial Computer (IPC - Pentium 266) with 14 scales connected on serial commports (RS422-4 wire connection).

We are handling 350 words (Read) and 62 words (Write) with 6 COMMAND LISTS (4 command list / 2 command list write). We are using FC (function Code) 16 to write and 4 to read registers. Example::

PORT SLAVE FC Start Address Destination Add. CNT
1 1 4 43 0 96
1 1 16 200 110 62

Almost transfer values are floating point (IEEE 754 values).

it receives this command in 8 seconds, besides
Scale STARTED immendiatly and SCALE STARTED status be returned in 3 seconds to IPC.

We would like to know if is it possible to improve communication performance?

Also if we connect all devices in RS-485, we will improve speed transaction?

Best Regards,
Mário R Domenici

By Michael E. Crossman on 22 July, 2002 - 3:08 pm


You may want to try setting up your write commands as conditional: ProSoft Manual Page 26 -> Conditional commands are executed only when a change in the block of data to be written is detected. Every time the Command List is scanned, the module will compare the data to be written against the data last written. If a change is detected in any value, bit or word, the entire data block controlled by the command is written.

Changing from RS-422 to RS-485 will not help increase speed.

Best regards,
Michael E. Crossman
PLCs Plus International, Inc.