I/O System and Bus Architecture
The NORD-10 was equipped with a common bus system for all external devices. The bus system was divided into groups, and a great deal of effort had been made to ensure that no device would be able to jam the bus system in the case of malfunction. Each group had its own controller which in addition to functioning as an electronic switch for the bus system, could also change priority for the whole group. All interconnections between the cards were done with multilayer printed circuit backwiring boards, and all input/output interface had the same standard form. The system could therefore be extended or reconfigured by plugging in new or shifting around the existing interface cards. The position of the device interface in the card rack determined the interrupt priority of the device. In direct memory access transfers the device would send a "request". The CPU would answer with a "grant" signal, which would be passed from device to device until it came to the device which initiated the "request", and transfer to the memory could take place. When two or more devices request a DMA request simultaneously to the CPU had the highest priority. One memory cycle later the next DMA along the chain would be allowed to send data, and so on, until a higher priority device again sent a request. This meant that many DMA devices could use the same bus system at the full data transfer rate. It was not necessary to establish a "master-slave" connection. The transfer was one 16-bit word/850 nanoseconds, or 2.2MB/s.
The printed backplane of the I/O bus was modular in groups of 8 interface slots. Interfaces for mass storages as disk, drum, magnetic tape, etc., were built with one interface card to be plugged at the appropriate place in the bus system, the remaining control cards (6-7) were placed in one of the backplane modules.
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