372596156

RIKEN Accel. Próg. Rep. 24 (1990)

111-5-25. High Speed Serial Data Link for PC-9801

J. Fujita

PC-9801 ftus

Personal computers are used in various fields of experiments such as CAD, network terminals or local data collection Systems of measurements. It is often necessary to transfer real time data at a high speed to remote systems. The high-speed data transfer can be realized by a conventional parallel wire method, but reąuires many wires. This problem can be solved by using serial data transfer, but the bit ratę must be increased by a factor equal to the number of bits per data fields. For example, an 8-bits parallel system operating at 10 MHz needs a serial data transmission capability of morę than 80 Mbps (8 MBytes/s). Serial data link, which consists of Am7968 transmitter and Am7969 receiver chips, possesses data throughput of morę than 100 Mbps (about 10 Mbytes/s). A data link interface board, using these chips and optical fiber components, is madę for a personal Computer PC-9801 and tested.

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Logic

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Fig. 1. A high speed data link interface board.

The błock diagram of the test Circuit is shown in Fig. 1. An optical fiber part of the data link sur-rounded by a broken linę is constructed on a smali separate board outside the interface board. The fiber ports are composed of an LED HFBR-1402, a light to voltage converter HFBR-2406, and their related circuits. Fast ECL signals between the data link chips and the fiber circuits are linked with four short RG-174/U cables.

The Am7968 transmitter has two operating modes of local and Cascade. The local modę consists of a single transmitter communicating with a single receiver over serial mediums. The Cascade modę consists of two or morę daisy-chained (Cascade) transmitters driving a equal number of receiv-ers over a single serial medium. Two Am7968 transmitters are cascaded to send a two-byte word over a single serial linę. In the Cascade operation two bytes of data are loaded in parallel through the Computer data bus simultaneously. The transmitter connected to the serial medium is referred to as the primary, and is operated in a local modę. The daisy-chained transmitter which supplies serial data to the primary is referred to as the upstream, and is operated in a cascade modę. Word data are written by a processor into transmitting FiFo memories of 512 bytes in size before transfer. Data transfer is began by address-ing a certain memory location. Once transmitting starts, the stocked quantity of data is continuously sent through the cascade over the serial medium without intervention of the processor in the manner of DMA operation until an empty flag of the FiFo memory locks the automatic cycle. The flag status is also sensed by the processor so that next data can be preset into the FiFo regardless of State of the serial medium. Idle time between transmitting data bytes is filled with SYNC bytes.

An Am7969 receiver monitors the serial link, selects only data bytes out of a serial pattem, and writes it into the receiving FiFo memory of 512 bytes in size, the contents of which are read by the processor through DATA L or lower 8 bits on the 16 bits data bus. As the cascade modę is not yet supported in the Am7969 receiver unlike the Am7968 transmitter, each data byte must be stored into the single FiFo memory in tum immediately after receipt. The access time of the receiving FiFo is 25 ns, half of that of the transmitting FiFo of 50 ns.

The P'iFo memories and data link control logie are allocated at even addresses from cOOOOH to cOOOaH in hexadecimal the locations of which are unused part of user’s ROM area in the Computer.

The serial data throughput of about 9 Mbytes/s is obtained in a self-back test done by mutually tying fiber input and output ports with a single optical fiber cable of 5 m long. This value is limited by an intemal logie of the chained transmitters; how-ever the test shows that the interface can serve sufficiently as a high-speed serial data link between remote Computer systems.