Programmable controller with CPU units and special-function modules and method of doubling up

A method of providing a programmable controller with a doubled-up structure, said method comprising the steps of:

• providing two CPU units each having a special-function module capable of carrying out a calculation process, one of said two CPU units being an active CPU in an active mode and carrying out a control process, the other of said two CPU units being a standby CPU unit in a standby mode;
• causing one of the special-function modules attached to said active CPU unit to be an active module and to carry out said control process together with said active CPU unit and causing the other of the special-function modules attached to said standby CPU unit to be a standby module;
• and
• causing said active CPU unit to be synchronized with said standby CPU unit by way of executing an I/O refresh, said I/O refresh including updating an I/O memory in said active CPU unit with data received from input units and copying data of said I/O memory in said active CPU to an I/O memory in said standby CPU unit;
• wherein the special-function module attached to the respective CPU unit obtains and compares results of said calculation process from both of the special-function modules and if said results do not match, the slower one of the special-function modules that is behind in said calculation process executes a correction process to further advance in said calculation process such that both of the special-function modules are equally advanced in said calculation process;
• and
• activating said standby CPU unit and said standby module into said active mode if an abnormal condition occurs in an active component selected from the group consisting of said active CPU unit and said active module.

A method of providing a programmable controller with a doubled-up structure, said method comprising the steps of:

• providing two CPU units each having a special-function module capable of carrying out a calculation process, one of said two CPU units being an active CPU in an active mode and carrying out a control process, the other of said two CPU units being a standby CPU unit in a standby mode;
• causing one of the special-function modules attached to said active CPU unit to be an active module and to carry out said control process together with said active CPU unit;
• causing the other of the special-function modules attached to said standby CPU unit to be a standby module;
• and
• causing said active CPU unit to be synchronized with said standby CPU unit by way of executing an I/O refresh, said I/O refresh including updating an I/O memory in said active CPU unit with data received from input units and copying data of said I/O memory in said active CPU to an I/O memory in said standby CPU unit;
• causing each of the CPU units to exchange data with the corresponding one of the special-function modules through a common memory;
• causing the two special-function modules to carry out said calculation process non-synchronously with said two CPU units to produce I/O data;
• preventing said I/O data from being written into said common memory while said calculation process is being carried out;
• and
• writing said I/O data into said common memory while the special-function modules are operating in synchronism with said CPU;
• wherein the special-function module attached to the respective CPU unit obtains and compares results of said calculation process from both of the special-function modules and if said results do not match, the slower one of the special-function modules that is behind in said calculation process executes a correction process to further advance in said calculation process such that both of the special-function modules are equally advanced in said calculation process.