|
Hmmm. Yes, the IBM 5150 was strictly for business. Only they would be mad enough anyhow.
I was thinking of the chips. All CPU chips had been 8 bit, with the exception of Motorala's 68000, which came out in 1979 iirc. The 68008 was the 8 bit version of that. Motorola got stuck in building 16 bit versions of cpu support chips, which you needed. Think of the Address and data buses: the ram chips were all address bus pins with one data. 11 address lines = 2048 = 2k; Then you needed lines for o/p, +,-, /CS, &/WE or translating, = output(1 bit of ram), positive, negative, chip select & Write Enable. The '/' meant they are active low. That was your 16 pins allocated, and you were done in 1981 or early 1982. Wider/bigger chip sizes came inside a year or two. And each of these TTL chips had a 2k pullup resistor on each input, consuming 2.5 milliamps if driven low. TTL outputs could only sink 20mA, so your fan-out(=how many chips you could drive) was 8 max and even then the levels were borderline. The cpu buses were good for 40mA (fan-out of 16)which was better. But pulling the ram down was over half an amp sunk through the chip! Memory controllers were nightmarish in the early days. CMOS only did 1Mhz then, and had a fanout of 1. It didn't have outputs, really, but variable resistors :-P.
Last edited by business_kid; 01-20-2020 at 12:58 PM.
|
