Xref: utzoo comp.dcom.modems:6436 comp.sys.ibm.pc.hardware:677 comp.sys.ibm.pc.misc:546 comp.os.msdos.programmer:397
Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!zephyr.ens.tek.com!gvgpsa!gold!grege
From: grege@gold.GVG.TEK.COM (Greg Ebert)
Newsgroups: comp.dcom.modems,comp.sys.ibm.pc.hardware,comp.sys.ibm.pc.misc,comp.os.msdos.programmer
Subject: Re: Setting and reseting DTR through the COM port
Keywords: DTR , COM
Message-ID: <1238@gold.GVG.TEK.COM>
Date: 6 Aug 90 21:06:03 GMT
References: <10038@hubcap.clemson.edu>
Followup-To: comp.dcom.modems
Organization: Grass Valley Group, Grass Valley, CA
Lines: 7

In article <10038@hubcap.clemson.edu> rmorris@hubcap.clemson.edu (Robert Morris) writes:
>I need to turn on a box which is hooked to a serial port and is activated by
>setting or resetting the DTR.  How does one do this?  I've seen software that   

Your serial port is on either 2F0-2F8, or 3F0-3F8. Use DEBUG to find out. 
Bit 0 of 2F4 (or 3F4) controls the state of DTR. Writing a '0' will put out
-12v on DTR; writing a '1' will put out +12v. Have fun.