First things first: Olin, I apologize for the un necessary personal criticism. BAJ> Which archive are you searching? Here's a post I wrote on the subject that BAJ> was easily found in the jkeyzer archive: BAJ> http://www.infosite.com/~jkeyzer/piclist/2002/Jan/1069.html I wasn't asking about RMW operations. BAJ> That's a bit harsh. Yes. It was. See apology above. I won't go into my bad day excuses.. but be aware they exist :) BAJ> We see so many folks come through here asking questions BAJ> without a lick of consultation with the datasheet and/or the archive that BAJ> it becomes a natural defense mechanism. It was unclear if you had done so. I know, I see them too, but I thought I had explained my original question well enough to avoid it being mis interpreted as a r-m-w question. Drew>It *has* been beaten to death over the course of several years. Maybe Drew>you've just missed the posts. Try searching for "RMW" or "R-M-W". RMW has been beaten to death. I was refering to whether or not a READ ONLY operation on a port destroyed the data held in the output latches. Drew>The 16C71 datasheet covers this problem briefly in the I/O Port section. Drew>It's on DS30272A-page 30. Drew>However, to be fair, Microchip does recommend researching the I/O ports Drew>further (from the 16F877 datasheet): Drew>"Additional information on I/O ports may be found in the Drew>PICmicro Mid-Range Reference Manual, (DS33023)." I have just read the IO sections of the data sheet, it does go into RMW quite nicely including : "The PORT register is the latch for the data to be output. When the PORT is read, the device reads the levels present on the I/O pins (not the latch). This means that care should be taken with read-modify-write commands on the ports and changing the direction of a pin from an input to an output." I was assuming that a read only operation would not affect the data in the port latch- although it doesnt say that specifically- but the last bit "care should be taken ... changing the direction from an input to an output" just confuses things further- or do they mean doing a RMW operation and then changing TRIS, in which case the caution doesnt really apply? -- Jon Baker -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads