Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!STL-HOST1.ARPA!SNELSON From: SNELSON@STL-HOST1.ARPA Newsgroups: comp.dcom.modems Subject: EIA Flow Control Message-ID: <[STL-HOST1.ARPA]Mon,.29.Feb.88.11:55:52.CST.SNELSON> Date: 29 Feb 88 17:55:00 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 142 The following is quoted for EIA STANDARD RS-232-C Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Interchange. In the forward it warns that a revised version of ISO International Standard 2110-1972 introduces some pin assignment incompatibilities with RS-323-C. Section 4.4 Circuit CA - Request to Send (C.C.I.T.T. 105) [Recommendation V.24] Direction: TO data communications equipment This circuit is used to condition the local data communications equipment for data transmission and, on a half duplex channel, to control the direction of data transmission of the local data communication equipment. On one way only channels or duplex channels, the ON condition maintains the data communication equipment in the transmit mode. The OFF condition maintains the data communication equipment in a non-transmit mode. On a half duplex channel, the ON condition maintains the data communication equipment in the transmit mode and inhibits the receive mode. The OFF condition maintains the data communication equiment in the receive mode. A transition form OFF to ON instructs the data communication equipment to enter the transmit code [possibly this should read mode?-a printer error?] (see Section 6.8). The data communication equipment responds by taking such action as may be necessary and indicates completion of such actions by turning ON Circuit CB (Clear to Send), thereby indicating to the data terminal equipment that data may be transferred across the interface point on inter- change Circuit BA (Transmitted Data). A transition from ON to OFF instructs the data communication equipment to complete the transmission of all data which has previously transferred across the interface point on interchange Circuit BA and then assume a non-transmit mode or a receive mode as appropriate. The data communications equipment responds to this instruction by turning OFF Circuit CB (Clear to Send) when it is prepared to again respond to a subsequent ON condition of Circuit CA. NOTE: A non-transmit mode does not imply that all line signals have been removed from the communication channel. See section 6.8. When Circuit CA is turned OFF, it shall not be turned ON again until Circuit CB has been turned OFF by the data communication equipment. An ON condition is required on Circuit CA as well as on Circuit CB, Circuit CC (Data Set Ready) and, where implemented, Circuit CD (Data Terminal Ready) whenever the data terminal equipment transfers data across the interface on interchange Circuit BA. It is permissible to turn Circuit CA ON at any time when Circuit CB is OFF regardsless of the condition of any other interchange circuit. Circuit CB - Clear to Send (C.C.I.T.T. 106) Direction: FROM data communication equipment Signals on this circuit are generated by the data communication equipment to indicate whether or not the data set is ready to transmit data. The ON condition together with the ON condition on interchange circuits CA, CC and, where impletment, CD, is an indication to the data terminal equipment that signals presented on Circuit BA (Transmitted Data) will be transmitted to the communication channel. The OFF condition is an indication to the data terminal equipment that it should not transfer data across the interface on interchange Circuit BA. The ON condition of Circuit CB is a response to the occurrence of a simultaneous ON condition on Circuits CC (Data Set Ready) and Circuit CA (Request to Send), delayed as may be appropriate to the data communications equipment for establishing a data communication channel (including the removal of the MARK HOLD clamp from the Received Data interchange circuit of the remote data set) to a remote data terminal equipment. Where Circuit CA (Request to Send) is not implemented in the data communication equipment with transmitting capability, Circuit CA shall be assumed to be in the ON condition at all times, and circuit CB shall respond accordingly. Section 6.8 The turning ON of circuit CA (Request to Send) does not necessarily imply the turning ON of a line signal on the communication channel. Some data sets might not have a line signal as it is understood in this standard, e.g. the signal can be modified digital base-band signal. Conversely, in data sets which do transmit a "line signal", the turning OFF of Circuit CA does not necessarily command the removal of that line signal from the communication channel. On a duplex channel, the data set might autonomously transmit a training signal to hold AGC Circuits or automatic equalizers in adjustment, or to keep timing locked (synchronized) when circuit CA is OFF. It is not within the scope of this standard to specify in detail what occurs on the communication channel (line) side of the data communication equipment. Therefore the definition for Circuit CA uses the terminology "assume the transmit mode" intentionally avoiding reference to "carrier" or "line signals". However, the continued requirement for multipoint systems is recognized. Data sets intended for this type of operation should permit the sharing of a communcation channel by more that one data set transmitter and should, when in a non-transmit mode, place no signal on the communcation channel which might interfere with the transmission from another data set in the network. end of quote. There are several related standards RS-269 RS-334 RS-363 RS-366 Interface Between Data Terminal Equipment and Automatic Calling Equipment for Data Communication Equipment. RS-404 RS-422 RS-423 RS-449 I personally think it is a bad practice to tie two or more drivers from one interface to a single receiver on another interface. XON/XOFF has a major problem in that it does not specify how soon the transmitting device has to stop sending upon receipt of an XOFF command. And there are several permutations of who is issuing XOFF and to whom and in a multi-link connection it is sure not clear what, or when, or who should stop sending and intercept the XOFF command before it shuts everything down. With all the possible delays, including propagation, getting XOFF to where the stoppee wants a device or protocol to stop sending, XOFF in band may not be a viable solution. I see on some of the other mailing lists some real sharp people addressing flow control. But I don't think they want to worry about the electrical interface. One additional thought. Some of this cutrate dial up service may be 32kbps voice or less. This may be fine for voice and some low speed data, but if you are trying to move higher that 4800 bps data over one of these cheepoes you are going to be in a world of hurt. Regards, Steve