This option provides 256 Kbytes of additional memory. Each 256-Kbyte memory DS25 Modell B 500W PSU option occupies one option slot. This daughter module resides on the system module, giving the Professional 380 1 Mbyte of memory without using an option slot. This option may also be used on the Professional DS25 Modell B 500W PSU. Systems 1.153 33-Mbyte, 5.25-in hard-disk drive and controller for the Pro- fessional. Features a DS25 Modell B 500W PSU 5-Mbit/sec transfer rate and an average access time of 49 milliseconds. The controller module occu- pies one option slot. 20-Mbyte, 5.25-in half-height form factor hard-disk drive and controller for the Professional. Features a 5-Mbit/sec transfer DS25 Modell B 500W PSU rate and an average access time of 49 milliseconds. The con- troller module occupies one option slot.