Internal devices use a 50 pin dual inline connector and a flat ribbon cable. Multiple connectors are usually attached to it in increments of around 12 inches (yet they might be farther apart). You can use any connector on the cable and in any order.
External devices use a different cable and connector for obvious reasons. The cable is usually consisted of 25 twisted pair wires, so you get a total of 50 wires. Each external device has two connectors on it's back side, they are connected to each other pin for pin and are used to connect the next device in the daisy chain or to connect the terminator. This means that you need a cable for each additional external device.
The standard SCSI connector has 50 pins in two rows and is termed as the type A connector from the SCSI specifications, this connector looks like a Centronics printer connector yet has 50 pins instead of 36. Some devices use a regular 25 pin D-shell connector, this connector is good, but can only handle single ended SCSI because they don't have enough connections.
SCSI-2 specified a new cable for wide SCSI, since the original Type A cable does not have enough connections on it. The SCSI-2 specifications indicates the use of a second cable described as the B cable, yet most developers did not follow this path and use a similar cable known as the type P cable. This cable has a 68 pin connector arranged in two rows of 34 pins just like the B cable. The difference is in the pin assignments on the connector. The B cable has since been abandoned and the standard is now the P cable.
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