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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.
Modbus Command Overview
| Code | Bits | Target | Count | Direction | Name in Spec | libmodbus Function Name |
|---|---|---|---|---|---|---|
| 01 | 1 | DO | many | read | Read Coils | read_coil_status |
| 02 | 1 | DI | many | read | Read Discrete Inputs | read_input_status |
| 03 | 16 | AO | many | read | Read Holding Registers | read_holding_registers |
| 04 | 16 | AI | many | read | Read Input Register | read_input_registers |
| 05 | 1 | DO | one | write | Write Single Coil | force_single_coil |
| 06 | 16 | AO | one | write | Write Single Register | preset_single_register |
| 15 | 1 | DO | many | write | Write Multiple Coils | force_multiple_coils |
| 16 | 16 | AO | many | write | Write Multiple registers | preset_multiple_registers |
More information sources:
Wago 750-342
Setting the IP with bootp
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()
See also: Wago manual p.82ff and bootptab
Libmodbus
Installation
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 and 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.
Wago Adressing in Libmodbus
Bit-based access
- Bit-based function codes are # 1,2,5,15
- The first digital input and output bit has address 0
- The 2nd digital input and output bit has address 1
- Addressing of inputs and outputs are independend (ie. there exists DI 0 and DO 0)
- Each input/output bit consumes one member the
uint8_t dest[]array. I.e. each bit consumes one byte.
Register-based access
- Register-based (16bit) function codes are # 3,4,6,16
- The first analog input and output bit has address 0
- Contrary to the Wago manuals the 2nd analog input and output bit has address 1
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