Modbus TCP

Collected informations for using Modbus on TCP/IP. Libmodbus is used as master on any Linux computer (Sheevaplug). Wago fieldbuscouplers 750-342 (or alternatively 750-841) are used as slaves.

Code	Target	Count	Direction	Name in Spec	Libmodbus 3 fkt name	libmodbus 2 fkt name
01 0x01	DO	many	read	Read Coils	modbus_read_bits	read_coil_status
02 0x02	DI	many	read	Read Discrete Inputs	modbus_read_input_bits	read_input_status
03 0x03	AO	many	read	Read Holding Registers	modbus_read_registers	read_holding_registers
04 0x04	AI	many	read	Read Input Register	modbus_read_input_registers	read_input_registers
05 0x05	DO	one	write	Write Single Coil	modbus_write_bit	force_single_coil
06 0x06	AO	one	write	Write Single Register	modbus_write_register	preset_single_register
15 0x0f	DO	many	write	Write Multiple Coils	modbus_write_bits	force_multiple_coils
16 0x10	AO	many	write	Write Multiple registers	modbus_write_registers	preset_multiple_registers
23 0x17	AI/AO	many	both	Wrt/Rd Multiple registers	modbus_write_and_read_registers

Sources Spec Manpage

More information sources:

• http://www.anybus.de/technologie/modbustcp.shtml
• http://www.modbus.org/docs/PI_MBUS_300.pdf
• http://jamod.sourceforge.net/kb/protocol.html

Basic principle: The bootp client (750-342) sends its MAC address to the bootp server (eg. a slackware linux computer). The server in turn provides the client with an IP address.

Example: To provide the client host with the name etb-130 who has the MAC address 0030DE010A2C with the IP address 192.168.132.130 and an appropriate subnet mask, add the following line to /etc/bootptab:

etb-130:ht=1:ha=0030DE010A2C:ip=192.168.132.130:sm=255.255.255.0

Then, on the server machine, start the bootp daemon by uncommenting the following line in /etc/inetd.conf

bootps  dgram   udp     wait    root    /usr/sbin/bootpd        bootpd

…and restart the inetd:

/etc/rc.d/rc.inetd restart

Now connect the Client with the server eg over crosslink cable and boot the client.

The 750-342 fetches its IP from the bootp server only when bootp is enabled. Otherwise it uses the IP saved in its EEPROM. Pseudcode:

ipNumber= bootpEnabled ? getIpNumberFromBootpServer() : getIpNumberFromEeprom()

Web access to 750-342 is admin / wago

See also: Wago manual p.82ff and bootptab

See http://libmodbus.org/site_media/html/libmodbus.html

Download the source and use the Slack build script.

Note: You could get the latest devel release with git clone http://github.com/stephane/libmodbus.git, but this has no configure script, so we use the lataest stable release instead.

Download v 2.0.3 from here

Libmodbus has the unpleasant behaviour, that it prints error messages to stdout instead of stderr. If you plan to build your application like a filter which commuicates over stdin / stdout to the world, than you will get problems, in that libmodbus may interfere your protocoll or whatever with error messages. Therfore we patch the source a little bit and are writing error messages to stderr instead of stdout. Look for the function error_treat in modbus/modbus.c and replace the printf function with fprintf:

static void error_treat(modbus_param_t *mb_param, int code, const char *string)
{
        printf("\nERROR %s (%d)\n", string, code);

static void error_treat(modbus_param_t *mb_param, int code, const char *string)
{
        fprintf(stderr, "\nERROR %s (%d)\n", string, code);

Now do the usual ./confiure; make; su; make install;. This installs the following objects to /usr/local/:

lib/libmodbus.so
lib/libmodbus.la
lib/libmodbus.so.2.0.0
lib/pkgconfig/modbus.pc
lib/libmodbus.so.2
include/modbus/modbus.h

Then do ldconfig to update the library system info.

• Bit-based function codes are # 1,2,5,15
• The first digital input and output bits have both address 0
• The 2nd digital input and output bits have both address 1
• Addressing of inputs and outputs are independent (ie. there exists DI 0 and DO 0)
• Each input/output bit consumes one member of the uint8_t dest[] array. I.e. each bit consumes one byte.

• Register-based (16bit) function codes are # 3,4,6,16
• The first analog input and output register have both address 0
• Contrary to the Wago manuals the 2nd analog input and output registers have address 1

• Access to digital inputs and outputs is also possible with the register-based function codes
• Digital input and output addresses are automatically appended after the analog addresses. This means the digital addresses are dependend of the number of the analog devices.
• Each digital IO consumes on bit in the uint16_t dest[] array