Apur-ee.105 net.general,NET.general,net.periphs utzoo!duke!cincy!pur-ee!mike Fri Jun 19 16:40:15 1981 DMC hardware fix The DEC dmc11 interprocessor link board M8200 has some hardware bugs which showed up in the process of bringing up our local network on our first vax 11780. The problem manifests itself as a bushang condition when running a DMC with other DMA devices transfering further down the bus from it (further from the UBA or processor). The problem has three seperate causes - 1. a race condition in the NPG recognition circuit, 2. an RC delay network in the NPG circuit which cannot handle back-to-back NPGs on the bus, and 3. an NPR request flag controlled by the micromachine internal to the DMC which does not drop fast enough after receiving NPG (as compared to the usual random logic arbitration circuit). To cure these problems, refer to page D15, sheet 16 of 22 of the engineering drawings for the DMC11 (Dec print set order # MP00076). You will find a resistor, R31, a capacitor, C110, and a diode, D4 on the left hand side of the page. Remove these components. In their place insert a 7404 inverter chip in the spare chip location E130 in the upper left hand corner of the board. Run a wire from pin 1 of this 7404 to the former input side of the RC delay circuit you just remover (the lefthand hole the resistor was in). Run another wire from the righthand resistor hole to pin 8 of the 7404 you just inserted. Connect pins 2 to 3, 4 to 5, 6 to 13, and 12 to 11, and 10 to 9. Connect pin 14 of the 7404 to +5volts and pin 7 to ground (use the leads of bypass cap E100 as connection points). Remove the 7408 in location E105 and replace it with a 74s08. Remove the 7400 in location E106 and replace it with a 74s00. This completes the modification. We have had one unit out of 14 where this failed to cure the problem, or the timing is rather critical and apparently varies enough with different chips elsewhere in the circuit to cause problems on a few boards. We simply left that board on an 11/45 where the faster bus ensures that (statistically speaking) it works reliably. The only solution to this problem without hacking at the hardware is to put the DMC at the end of the bus, but this makes it the lowest priority dma device, which is not good for a dmc, and limits you to one dmc per UBA. Mike Marsh pur-ee!mike ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.