My Site

Peripheral Addressing

First all the peripherals are connected to a controller. The controllers are connected to the computer. The controller can be in a box all by itself. A head high box. The controller could be in the peripheral, most commonly the first in a string of the same peripheral.

All the peripherals in the system have an address. The address is three hex characters long. The first character is the channel. This is determined by where it plugs into the computer. The second is determined by the controller. The third is for the individual peripheral. Either the peripheral determines its own number or it is based on the order the peripheral is plugged into the controller.

Disk drives in particular let the operator easily change their address. They had plugs, with magnets in them, the operator could change and the drive read the magnets to decide what the address was. We had a bank of 5 2314s and each had a 3 inch diameter hole that you could put the plug in. The plugs has something like 140-144 and you could swap them around.

By convention, not requirement, the printer and card reader/punch were on channel 0, the multiplexer, you could have more than one peripheral active, and they were 00C for the card reader, 00D for the card punch, reader and punch were two sides of the same box, and 00E for the printer. Disks were on channel 1, you could not multiplex, only one disk at a time could transfer data. Tapes were on channel 2, also not multiplex.

Chanel 0 was a little different. I could multiplex devices. The devices were slow and you could transfer data to multiple of them at once. The other channels had faster devices and only one could be active at a time. For disks in particular they could use the channel for the seek address, disconnect for the relatively long time it took to seek, and later rotate, and reconnect when they were ready to transfer date. So, if you had a string of drives, one could be seeking and another could be transfering data. Tape drives could disconnect for something like rewind, but normally for a read/write a drive stayed connected for the entire time. You could watch a string of tapes and see them read/write one after another. Tape sorts, a sort using tape as the intermediate storage, were especially entertaining to watch.