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Appendix A - MODBUS RTU Protocol
APPENDIX A -
MODBUS RTU PROTOCOL
A.1.
Introduction
This document summarizes the serial link protocol to / from the DIGITAL SOFT STARTER(ASTAT-XT).
Features:
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RS485 Hardware.
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Asynchronous serial link.
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Half duplex.
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Format: Modbus RTU Mode (Remote Terminal Unit Mode).
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Binary
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Each character contains 11 bits:
- One start bit
-8 data bits, least significant bit sent first.
- 1 Parity bit. Even / Odd / No can be selected.
- 1 Stop bit if Parity is used, 2 stop bits if Parity is not used.
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Cyclical Redundancy Check (CRC), 16 bits.
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Baud Rates: 1200 / 2400 / 4800 / 9600 bits per second.
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Response time of the ASTAT-XT:
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Normally, 4ms <= time response <= 40mS.
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For a long response, time response <= 200mS.
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It is not recommended to transmit to the ASTAT-XT at a rate higher than once per second because this can slow
down ASTAT-XT response times.
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After storing parameter settings, there is a 1 sec. time period during which transmission to the same ASTAT-XT is
forbidden.
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Broadcast commands: not supported.
Notes:
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It is recommended that you connect 120 OHM resistors to both ends of the serial link.
*
You must turn OFF (and ON again) power to the controller after changing Baud_Rate, Parity_Check or
Serial_Link_No (Slave Address). These parameters can only be modified manually and not through the Serial Link.
WARNING
You must connect earth to the ASTAT-XT earth screw before connecting serial link wires.
Ignoring this instruction may result in permanent damage to the Serial Link Hardware
and might result in death or serious injury.