HELLER%cs.umass.edu@RELAY.CS.NET (Stride 440 User) (10/10/86)
At one point we had RK07 disks & a DR11-B (old DMA parallel interface, preceded the DR11-W) and got DATA LATE errors on the RK07 when DMA transfers happened on the DR11-B. We solved the problem by re-aranging things on the Unibus: RK07 first, then DZ's and other "programmed I/O" boards (i.e. DR11-C/K), then EtherNet board, and finnally the DR11-B last. The problem is caused by the fact the the DR11-W or DR11-B is not really very smart about "bus hogging". Usually it is connected to some dumb I/O device (like a Grinnell GMR-27 or GMR-270), which will eat 16 bit words at its own pace and once the DR11-W/B starts up a DMA transfer it grabs the bus and keeps it until the transfer completes. The RK07 driver and controller tries to be smart by freeing up the bus when it does not really need it. The problem happens when an I/O request is setup with the RK07 and the controller needs to wait on rotational latency. It frees up the bus while waiting for the sector to come around, meanwhile the DR11-W/B gets a DMA I/O request and grabs the bus. The sector comes around, but the bus is busy. The controller yells DATA LATE and tries for the next time around... Putting the RK07 ahead of the DR11-W/B on the unibus gives it a higher priority, which means it gets to grab the bus from the DR11-W/B when it needs to. Usually the DR11-W/B can wait (the RK07 only transfers 256 words (= 512 bytes = 1 sector) at a time anyway). You might still get DATA LATES, but not as often. We have our own driver for our DR11-B and DR11-W devices (a Grinnell GMR-27 and a Grinnell GMR-270), which dates back to VMS 0.5 (yes we have an OLD 11/780 and an OLD GMR-27). We use the same driver for both the DR11-B and the DR11-W (which is operating in DR11-B mode). The DR11-B is on a 780 and the the DR11-W is on a 750. The DR11-B is connected to the GMR-27 and the DR11-W to a GMR-270. Neither machine has an RK07 on it anymore (although they both used to). Both systems do have DMA EtherNet boards and the 780 also has a COMTAL VISION One/20 on it (also DMA). Robert Heller ARPA: Heller@UMass-CS.CSNET BITNET: Heller@UMass.BITNET BIX: Heller GEnie: RHeller