Schneider Electric 174 CEV manual Read Discrete Output Status

Modbus Plus to Ethernet Bridge

Table 2 Summary of Modbus Data Access Commands

Function Code (Decimal)		Command Name
1		Read Discrete Output Status (0xxxx)
2		Read Discrete Input Status (1xxxx)
3		Read Output Register (4xxxx)
4		Read Input Register (3xxxx)
5		Force Single Coil (0xxxx)
6		Preset Single Register (4xxxx)
7		Read Exception Status
8	Subfunction 21	Get/Clear Network Statistics
15		Force Multiple Coils (0xxxx)
16		Preset Multiple Registers (4xxxx)
17		Report Slave ID

Devices which use the Modbus protocol accept incoming requests for data or statistics. They reply by returning either a normal or an exception response.

The Modbus function code in the request from an originating node always contains a most significant bit value of 0 (zero). In a normal Modbus response, the function code of the originating request is echoed, followed by any requested data or statistics. In an exception response, the most significant bit is set to 1 (one) to indicate an error, and the next byte contains an exception code showing the reason for the error.

Table 3 summarizes the Modbus exception codes which may be returned by the bridge to the originating node. For further information see the Modbus Protocol Reference Guide, publication PI--MBUS--300.

Table 3 Summary of Modbus Exception Codes

Exception Code (Hexadecimal)	Meaning
01	Illegal function
02	Illegal data address
03	Illegal data value
04	Unexpected failure during command execution
05	ACK ---- Requested operation is in progress
06	BUSY ---- Not ready to perform requested operation
07	NAK ---- Requested operation rejected
08	Could not open Modbus Plus path
09	Error during Modbus Plus send
0A	Error during Modbus Plus reply

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