Communication system and transmission apparatus

A communication system comprising a first communication device and a second communication device that are configured to perform serial communication with each other, either one device of the first communication device and the second communication device including a direct-current power source, the first communication device and the second communication device being connected to each other via a communication line that allows electric current supplied from the direct-current power source to flow therethrough from the one device to another device and via a reference line that allows the electric current passed through the communication line to flow therethrough from the other device to the one device, wherein

• the first communication device comprises:
• a first transmission circuit comprising a first transmission photocoupler, a conductive/non-conductive state of the first transmission photocoupler being controlled in accordance with data to be transmitted to the second communication device, the first transmission circuit being configured to control a conductive/non-conductive state of a transmission path including the communication line and the reference line in accordance with the conductive/non-conductive state of the first transmission photocoupler;
• and
• a first reception circuit comprising a first reception photocoupler, a conductive/non-conductive state of the first reception photocoupler being controlled in accordance with the conductive/non-conductive state of the transmission path, the first reception circuit being configured to detect the conductive/non-conductive state of the first reception photocoupler, thereby receiving data transmitted by the second communication device,
• the second communication device comprises:
• a second transmission circuit comprising a second transmission photocoupler, a conductive/non-conductive state of the second transmission photocoupler being controlled in accordance with data to be transmitted to the first communication device, the second transmission circuit being configured to control the conductive/non-conductive state of the transmission path in accordance with the conductive/non-conductive state of the second transmission photocoupler;
• and
• a second reception circuit comprising a second reception photocoupler, a conductive/non-conductive state of the second reception photocoupler being controlled in accordance with the conductive/non-conductive state of the transmission path, the second reception circuit being configured to detect the conductive/non-conductive state of the second reception photocoupler, thereby receiving data transmitted by the first communication device,
• the second reception photocoupler is included in the transmission path, and
• the first transmission photocoupler and the second reception photocoupler are in opposite conductive/non-conductive states.