PC, XT and AT 8 bit BUS Structure
┌────────┐ Ground ─┤B1 A1├─ -I/O CH CHK (NMI) +Reset DRV ─┤B2 A2├─ +Data 7 +5V ─┤B3 A3├─ +Data 6 +IRQ2 ─┤B4 A4├─ +Data 5 -5V ─┤B5 A5├─ +Data 4 +DRQ2 ─┤B6 A6├─ +Data 3 -12V ─┤B7 A7├─ +Data 2 -CARD SLCTD ─┤B8 A8├─ +Data 1 +12V ─┤B9 A9├─ +Data 0 Ground ─┤B10 A10├─ +I/O CH RDY -MEMW ─┤B11 A11├─ +AEN -MEMR ─┤B12 A12├─ +Address 19 -IOW ─┤B13 A13├─ +Address 18 -IOR ─┤B14 A14├─ +Address 17 -DACK3 ─┤B15 A15├─ +Address 16 +DRQ3 ─┤B16 A16├─ +Address 15 -DACK1 ─┤B17 A17├─ +Address 14 +DRQ1 ─┤B18 A18├─ +Address 13 -DACK0 (MREF) ─┤B19 A19├─ +Address 12 CLK ─┤B20 A20├─ +Address 11 +IRQ7 ─┤B21 A21├─ +Address 10 +IRQ6 ─┤B22 A22├─ +Address 9 +IRQ5 ─┤B23 A23├─ +Address 8 +IRQ4 ─┤B24 A24├─ +Address 7 +IRQ3 ─┤B25 A25├─ +Address 6 -DACK2 ─┤B26 A26├─ +Address 5 +TC ─┤B27 A27├─ +Address 4 +ALE ─┤B28 A28├─ +Address 3 +5V ─┤B29 A29├─ +Address 2 +OSC ─┤B30 A30├─ +Address 1 Ground ─┤B31 A31├─ +Address 0 └────────┘ AT 16 Bit BUS Extension ┌────────┐ -MEM CS16 ─┤D1 C1├─ SBHE -I/O CS16 ─┤D2 C2├─ Address 23 IRQ10 ─┤D3 C3├─ Address 22 IRQ11 ─┤D4 C4├─ Address 21 IRQ12 ─┤D5 C2├─ Address 20 IRQ15 ─┤D6 C6├─ Address 19 IRQ14 ─┤D7 C7├─ Address 18 -DACK0 ─┤D8 C8├─ Address 17 DRQ0 ─┤D9 C9├─ -MEMR -DACK5 ─┤D10 C10├─ -MEMW DRQ5 ─┤D11 C11├─ Data 8 -DACK6 ─┤D12 C12├─ Data 9 DRQ6 ─┤D13 C13├─ Data 10 -DACK7 ─┤D14 C24├─ Data 11 DRQ7 ─┤D15 C15├─ Data 12 +5V ─┤D16 C16├─ Data 13 -Master ─┤D17 C17├─ Data 14 Ground ─┤D18 C18├─ Data 15 └────────┘ - pin numbering starts from the rear of the machine Signal Description A0-A19 Address Bits 0-19 allow access to 1Mb memory and 64K of port addresses. A17-A23 Address Bits 17-23 allow access from 1Mb memory to 16Mb AEN Address Enable; When active the DMA controller has control of the Address and Data BUS as well as the MEMR/MEMW lines. When inactive the CPU has control of these lines ALE Address Latch Enable (output); used to latch addresses from the CPU. Forced active during DMA cycles. CARD SLCTD Card Selected; activated by cards in the XT's slot 8 CLK System clock signal (actual BUS speed) D0-D7 Data bits 0-7 for I/O to memory and I/O DACK0-DACK3 DMA Acknowledge for channels 0-3; used by the controller to acknowledge DMA requested. DACK0 is used for memory refresh (MREF) DRQ0-DRQ3 DMA Request 0-3; used by peripherals to get service from the DMA controller; Held active until the corresponding DACKx signal becomes active. I/O CH CHK I/O Channel Check; Generates a Non Maskable Interrupt I/O CH RDY I/O Channel Ready; pulled inactive my memory or I/O devices to lengthen memory or I/O cycles. Usually used by slower devices to add wait states. Should not be held inactive for more than 17 cycles. I/O CS16 I/O Chip Select 16 Bit; 16 bit I/O cycle IOR I/O Read; instructs an I/O device to drive its data onto the system BUS IOW I/O Write; instructs an I/O device to read data from the BUS IRQ2-IRQ7 Interrupt Requests 2-7; signals the CPU that an I/O device needs service (see 8259) MASTER Used with DRQ to gain control of system MEM CS16 Memory Chip Select 16 Bit; 16 Bit memory cycle MEMR Memory Read; this signal is driven by the CPU or DMA controller and instructs memory to drive its data onto the system BUS. Present on both PC and AT extension BUS MEMW Memory Write; this signal is driven by the CPU or DMA controller and instructs memory to read and store data from the system BUS. Present on both PC and AT extension BUS OSC Oscillator; 14.31818 MHz clock (70ns period); 50% duty cycle RESET DRV Reset Drive; used to reset system logic SBHE System BUS High Enable; activates data bits 8-15 on AT extension BUS TC Terminal Count; pulses when the terminal count for a DMA channel is reached - all ISA BUS signals use standard TTL levels - input and output are relative to the CPU