jkg@prism.gatech.EDU (Jim Greenlee) (11/01/90)
Can anybody tell me whether or not the 68000 family peripherals have open-drain DTACK outputs? I'm mainly interested in the 68681, 68230, and 68901. Unfortunately, the M68000 Family Reference Manual isn't real clear on this topic. For example, the 68681 DTACK is described as a "three-state active low open-drain output". The 68230 data section says that "a pullup resistor is required to maintain DTACK high between bus cycles", which implies (to me, anyway) that it is also open-drain. The 68901 DTACK is described only as an "active low, three-state output". It sure would simplify my life if they were all open-drain. Does anybody know for sure? Jim Greenlee -- Jim Greenlee - Instructor, School of ICS, Georgia Tech jkg@cc.gatech.edu Jryy, abj lbh'ir tbar naq qbar vg! Whfg unq gb xrrc svqqyvat jvgu vg hagvy lbh oebxr vg, qvqa'g lbh?!
fff@engmicroplex.com (Fred Fierling) (11/02/90)
In article <16185@hydra.gatech.EDU>, jkg@prism.gatech.EDU (Jim Greenlee) writes: > Can anybody tell me whether or not the 68000 family peripherals have > open-drain DTACK outputs? I'm mainly interested in the 68681, 68230, and > 68901. Unfortunately, the M68000 Family Reference Manual isn't real clear > on this topic. DTACK can be operated either way. As a three state output can you can drive DTACK when AS is asserted. During the cycle the output drives DTACK high (negate), then low (assert) to terminate. A pull up resistor is used to keep DTACK from rattling about when AS is negated. With an open collector or open drain output you just assert DTACK part way through the cycle and then let your pull up resistor negate it. You can take either approach. For systems with a high speed clock or that operate in a noisy environment the former should be more resistant to spurious DTACK's. -- Fred Fierling fff@microplex.com Tel: 604 875-1461 Fax: 604 875-9029 Microplex Systems Ltd 265 East 1st Avenue Vancouver, BC V5T 1A7, Canada