XT BUS

10/3/97

XT Bus - A peripheral bus implemented on the Original IBM-PC and PC-XT. The XT bus is a somewhat decoded extension of the signals from the eight bit 8088 CPU used in those machines and the signals from a few relevant peripheral chips. The XT bus generally had 8 slots for peripheral cards -- fewer in later machines. The slots are identical except that a single slot in the PC-XT has a Card Selected signal on a otherwise unused pin.

The XT-Bus uses a 62 pin (31 pins on each side) edge connector which has 20 Address Lines, 8 Data lines, 6 Interrupt lines, 4 sets of DMA lines (2 per DMA channels 1-3, DMA Acknowledge only for DMA0), 3 Grounds, 5 power lines (5, 5, -5, 12, -12 Volts), A Clock and 11 Control signals. The XT Bus was originally designed to run at 4.77MHz, but was later pushed to 6, 8 or even 10MHz in clones. The later AT, EISA, and VLB buses allow 8 bit XT bus cards to be used, generally at 8MHz. Even as of 1997, many modem cards and other simple cards are made with only the 8 bit XT bus connector.

After allowing for overhead operations such as card selection, and lost cycles due to the multiplexing of addresses and data on the 8088, the XT-Bus in a PC or PC-AT probably ran at less than 1 MegaByte Per Second throughput. In more modern designs it might manage 2MByte/s.

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