Path: utzoo!mnetor!uunet!vixie!paul From: paul@vixie.UUCP (Paul Vixie Esq) Newsgroups: comp.dcom.modems Subject: Re: EIA Flow Control Message-ID: <825@vixie.UUCP> Date: 23 Feb 88 16:31:16 GMT References: <8802222331.AA03700@ucbvax.Berkeley.EDU> Reply-To: paul@vixie.UUCP (Paul Vixie Esq) Organization: Vixie Enterprises, San Francisco Lines: 40 In <8802222331.AA03700@ucbvax.Berkeley.EDU> RAF@NIHCU.BITNET ("Roger Fajman"): ## As has been pointed out, there are many, many ways to make a null modem. ## The cross of 4 and 5 is 'best' when you have devices which implement ## the so called "EIA Flow Control" (aka hardware flow control). Here, ## CTS controls the flow from DTE to DCE, RTS controls the flow from ## DCE to DTE. In this scheme, there is no longer any correlation between ## the assertion of RTS and the reply with CTS. # #Interesting. What are some examples of devices that work that way? #I haven't run across any, while I have seen a lot that use DTR. The Telebit Trailblazer can work this way, if you ask it to. My Symmetric 375 can do it. The Bell Technologies ICC multiport serial card can do it. The Ann Arbor Ambassador can do it. Ungermann-Bass networked serial-port controllers have a special mode that does exactly this. #Does the RS-232C standard really cover this use of Request To Send? No. The standard says that RTS/CTS is for controlling the direction of data transfer on a modem that can only transmit one direction at a time. However, there are a lot of devices that can transmit in BOTH directions at a time, and there is a Real Need for 8-bit transparency. In such applications, the need for 8-bit transparency and full modem control (DTR/DCD/DSR) are much greater than the (non-existent) need for modem turn-around -- the modem doesn't turn around. I bought several "Modem Communications Bibles" to hopefully learn about how to make cables for a Bell Technologies ICC card, and none of them said any- thing about this "full duplex" use of RTS/CTS. It very desperately needs to become an official standard, since it is in very wide use as an unofficial one. In the Ungermann-Bass product line, there is a configuration menu item just for this. There is another for the RTS/CTS used in half-duplex modems. In short, Everybody Does It This Way, so could somebody please document it in an IEEE or ANSI spec somewhere? -- Paul A Vixie Esq paul%vixie@uunet.uu.net {uunet,ptsfa,hoptoad}!vixie!paul San Francisco, (415) 647-7023