Compose PLC Firewall™ (CM) Every Compose PLC system requires the use of a Firewall. The Compose PLC Firewall links all of the devices in the system and ensures reliable, predictable, performance. The Firewall also allows you to expand your lighting control system anytime you like. Required with every Compose PLC system Built-in RS232 interface (for use with CPLCOMM option) Optional time-clock Optional relay card * To be used with Lighhtolier Controls switches and dimmers