ECE385F Lab Pages - Lab 2 Prep Notes - ISA Signal Description & Timing Diagram

ISA Signal Descriptions

• SA19 to SA0
  System Address bits 19:0 are used to address memory and I/O devices within the system. These signals may be used along with LA23 to LA17 to address up to 16 megabytes of memory. Only the lower 16 bits are used during I/O operations to address up to 64K I/O locations. SA19 is the most significant bit. SA0 is the least significant bit. These signals are gated on the system bus when BALE is high and are latched on the falling edge of BALE. They remain valid throughout a read or write command. These signals are normally driven by the system microprocessor or DMA controller, but may also be driven by a bus master on an ISA board that takes ownership of the bus.

• AEN
  Address Enable is used to degate the system microprocessor and other devices from the bus during DMA transfers. When this signal is active the system DMA controller has control of the address, data, and read/write signals. This signal should be included as part of ISA board select decodes to prevent incorrect board selects during DMA cycles.

• SD15 to SD0
  System Data serves as the data bus bits for devices on the ISA bus. SD15 is the most significant bit. SD0 is the least significant bits. SD7 to SD0 are used for transfer of data with 8-bit devices. SD15 to SD0 are used for transfer of data with 16-bit devices. 16-bit devices transferring data with 8-bit devices shall convert the transfer into two 8-bit cycles using SD7 to SD0.

• -IOR
  I/O Read is driven by the owner of the bus and instructs the selected I/O device to drive read data onto the data bus.

• -IOW
  I/O Write is driven by the owner of the bus and instructs the selected I/O device to capture the write data on the data bus.

ISA Bus Timing Diagrams

             ________
AEN       __|        |_________________________________________
          _  ______________________________________________  __
SA(15:0)  _><______________________________________________><__
-SBHE
          ______________                                _______
-IOR/W                  |______________________________|
                                                _____________
SD(7:0)   -------------------------------------<_____________>-
 (READ)
                           __________________________________
SD(7:0)   ----------------<__________________________________>-
 (WRITE)