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USA BASED & SHIPS FROM AMAZON, BRAND NEW! The serial bus (IEC or CBM bus) is the standard interface for connecting external devices as disk drives or printers. Technically, it is a commodore-specific variant of the parallel IEEE-488/IEC-625-Bus, therefore it is often called IEC bus or CBM bus. This interface variant is not compatible with RS232, which is the standard for the serial interface for IBM-compatible PCs. Since serial ports are usually used only for the connection of two devices (e.g. RS232) the IEC bus is a serial bus. More than only one device can be connected, therefore most of the peripheral devices have loop-through connectors. Theoretically there are up to 31 devices possible (device numbers 0 to 31, 31 is reserved for UNTALK/UNLISTEN). However, the operating system of the C64 can use only the device numbers 4 to 30, because 0-3 are already used internally. Practically the upper limit is at around 8 devices. The reason for this is, that the chips (inverting driver 7406) in the C64 can handle a maximum load current of 40 mA. And any device, which is connected to the serial bus draws 5mA. In standard operation (LOAD, unmodified KERNAL-routines) the serial bus is capable to transmit 400 byte/second using a 1541 and approx. 650 bytes/second using an SD2IEC. With JiffyDOS and a 1541 approx. 2.4 kbyte/second are transmitted while JiffyDOS and a SD2IEC achieve 8.6 kbyte/second. The theoretic maximum is at about 20-25 kbyte/second.