nine-bit parallel bus; if the initiator is receiving data, it omits this step. The initiator then drops REQ low. If the target is to receive the data, it picks up the data and drops ACK low. If the target is to send the data, it puts the data on the bus and drops ACK low. When the initiator sees ACK low, if it is receiving data, it picks up the data from the data bus and raises REQ. When the target sees REQ high, it raises ACK high so the next transfer can take place. Up to 1.5 MB/s can be transferred on an SCSI bus this way, according to the original SCSI standard, but faster SCSI buses have been implemented and standardized.

9.5 Conclusions

Communications systems are among the most important I/O systems in a microcomputer. The microcomputer communicates with keyboards, displays, and typewriters, as well as with remote-control stations and other microcomputers, using the UART protocol. The microcomputer can be in a large computer system and have to communicate with other parts of the system using the SDLC protocol. It may be in a laboratory and have to communicate with instrumentation on an IEEE-488 bus. It may have to talk with other systems in a different protocol.

If you would like additional reading, we recommend the excellent Technical Aspects of Data Communication, by John McNamara. It embodies an exceptional amount of practical information, especially at the physical level, and also covers many of the widely used protocols. Motorola offers some fine applications notes on the SDLC protocol and its 6854 chip, such as MC6S54 ADLC, An Introduction to Data Communication, by M. Neumann. For the IEEE-488 protocol using the 68488 chip, the applications note Getting Aboard the 488-1975 Bus is very informative. These applications notes are well written and take you from where this book leaves off to where you can design systems using these protocols.

For more concrete information on the 68HC11, please consult the MC68HCHA8

HCMOS Single-Chip Microcomputer (ADI 1207). In particular, §5 describes the serial communication interface. As noted earlier, we have not attempted to duplicate the diagrams and discussions in that book because we assume you will refer to it while reading this book, and since we present an alternative view of the subject, you can use either or both views.

This chapter covered the main concepts of communications systems at the physical and link-control levels. You should be aware of these concepts so you can understand the problems and capabilities of specialists in this field. You should be able to handle the UART protocol - the simplest and most widely used protocol - and its variations,and you also should be able to use the SCI system in the 6812 and the ACIA chip, as well as the UART, in hardware designs. You should be able to write initialization rituals, interrupt handlers, and gadfly routines to input or output data using such hardware. Hardware and software tools like these should serve most of your design needs and prepare you for designing with the SDLC, IEEE-488, or SCSI interface protocol systems.