8155 +--------\/--------+ 1 -|pc3 vcc|- 40 2 -|pc4 pc2|- 39 3 -|timer in pc1|- 38 4 -|reset pc0|- 37 5 -|pc5 pb7|- 36 6 -|/timer out pb6|- 35 7 -|io/m pb5|- 34 8 -|/ce pb4|- 33 9 -|/rd pb3|- 32 10 -|/wr pb2|- 31 11 -|ale 8155 pb1|- 30 12 -|ad0 pb0|- 29 13 -|ad1 pa7|- 28 14 -|ad2 pa6|- 27 15 -|ad3 pa5|- 26 16 -|ad4 pa4|- 25 17 -|ad5 pa3|- 24 18 -|ad6 pa2|- 23 19 -|ad7 pa1|- 22 20 -|vss pa0|- 21 +------------------+ The 8155/8156 is a very old combination chip containing 256*8 RAM, a timer and 8+8+6 ports. The only difference between the 8155 and 8156 seems to be that the chip enable (pin 8) is active low on the 8155 and active high on the 8156.