control@almsa-1 (William Martin) (08/29/85)
What is the exact definition of a "null modem" cable? Does the term *only* mean that the connections to pins 2 and 3 on a DB25 RS232 connector on each end are interchanged, or does it also imply connections of other pins or the shorting or cross-connecting of certain pins (like 1, 7, 6, and 20)? Or is it a general term with no exact definition, except that it implies that the 2 & 3 connections are crossed? Will Martin ARPA/MILNET: wmartin@almsa-1.ARPA UUCP/Usenet: seismo!brl-bmd!wmartin
saltiel@cdstar.UUCP (Jack Saltiel) (08/29/85)
In article <195@almsa-1>, control@almsa-1 (William Martin) writes: > What is the exact definition of a "null modem" cable? > > Or is it a general term with no exact definition, except that it implies > that the 2 & 3 connections are crossed? > That's exactly right. Depends upon what modem functions you are trying to 'nullify' or imitate. The one that we use that usually (99%+) succeeds in allowing data to flow is: ----------------------- f 2 -------------------\ /------------------ 2 f X e 3 -------------------/ \------------------ 3 e m 4 ----\ /--- 4 m | | a 5 ----/ \--- 5 a l 6 ------\ /------ 6 l | | e 7 ---------------------------------------- 7 e | | 8 ------+ +------ 8 | | 20 ------/ \------ 20 2 = TD Transmit Data 3 = RD Receive Data 4 = RTS Request to Send 5 = CTS Clear to Send 6 = DTR Data Terminal Ready 7 = Signal Ground 8 = CD Carrier Detect 20 = DSR Data Set Ready Alternate arrangements have 4 & 5 crossed between the two end to allow cooperating hardware perform a RTS/CTS handshake. Other have that and/or DTR on the left tied to DSR & CD on the right & visa versa. This allows another kind of handshake. If you don't need the handshake, use the one drawn above. It will almost always allow data to pass. (PS: I may have RD & TD, RTS & CTS, and DTR & DSR reversed--this is from memory(at home) but just check an EIA chart first. It only matters if you're doing any hardware handshaking.) -- Jack Saltiel Cambridge Digital Systems {wjh12,talcott}!cdstar!saltiel "Here's to plain speaking and clear understanding." "I like a man who likes to talk."
phil@amdcad.UUCP (Phil Ngai) (08/30/85)
In article <195@almsa-1> control@almsa-1 (William Martin) writes: >What is the exact definition of a "null modem" cable? Does the term >*only* mean 1) that the connections to pins 2 and 3 on a DB25 RS232 >connector on each end are interchanged, 2) or does it also imply >connections of other pins or the shorting or cross-connecting of >certain pins (like 1, 7, 6, and 20)? > 3) Or is it a general term with no exact definition, except that it implies >that the 2 & 3 connections are crossed? The way I look at it, RS232 is not a standard, it is just a way of saying you are going to try to make two devices talk together. With whatever pins work. (ever try to hook up an NEC Spinwriter to a computer?) But in answer to your question, 3) is certainly right, 1) works for many DEC devices and 2) is sometimes needed. If you need 2), the most general case is as follows: 2-----3 3-----2 7-----7 4-- --4 | | 5-- --5 6-- --6 | | 8-- --8 | | 20- -20 -- "What really knocks me out, is a book that, when you've done reading it, you wish the author that wrote it was a terrific friend of yours and you could call him up on the phone whenever you felt like it." Phil Ngai (408) 749-5720 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!phil ARPA: amdcad!phil@decwrl.ARPA
hes@ecsvax.UUCP (Henry Schaffer) (08/30/85)
> What is the exact definition of a "null modem" cable? ... > > Will Martin > I don't believe that there is a standard wiring - but the standard definition is that the "null modem" (also called a "crossover" or sometimes a "modem eliminator") converts a DTE interface to a DCE and vice-versa (DTE-data terminal equipment, e.g. terminal. DCE-data communication equipment, e.g. modem.) To do this some pins are passed through, e.g. 1 and 7,; some are "crossed over", e.g. 2 and 3, and some are just fed back at each end, e.g.6 and 20 are connected together at each end and don't go through. Then vendors differ as to exactly what else they do. Glasgal Communications also connects 4,5 and 8 at each end, while Black Box Products connects 4 and 5 at each end and connects 8 to the 6 and 20 at each end. Black Box also passes through 11 and 12 crossed over. (This info is from their respective catalogs.) The one I use to connect a Model 100 to an IBM PC has the following connections: Male Female 1 1 2 3 3 2 4 5 5 4 6,20 8 7 7 8 6,20 Someone elso made it for me, so I can't say whether this is the minimum necessary wiring - but it does work. Note it isn't exactly either one of the above vendor's schemes. Also note that the genders of the connectors are not standard (although female-female is probably most common.) You have to specify what you want. --henry schaffer
taylor@ecsvax.UUCP (Steven Taylor) (08/30/85)
The main difference between a "null modem cable" and a "modem eliminator" in common usage of the terms is that a "modem eliminator" OFTEN also has the capability to provide a clock signal on pins 15 and 17. This is commonly used to connect two synchronous devices which require external clock (which would normally be supplied by the modem, hence the "eliminator.") Steven Taylor Distributed Networking Associates 119 Doncaster Lane Charlottesville, VA 22901 (804) 979-0656
gws@cbnap.UUCP (Gary W. Sanders (N8EMR)) (08/30/85)
In article <3209@amdcad.UUCP> phil@amdcad.UUCP (Phil Ngai) writes: >In article <195@almsa-1> control@almsa-1 (William Martin) writes: >>What is the exact definition of a "null modem" cable? Does the term >>*only* mean > > >>The way I look at it, RS232 is not a standard, it is just a way of saying >>you are going to try to make two devices talk together. With whatever >>pins work. (ever try to hook up an NEC Spinwriter to a computer?) > >>But in answer to your question, 3) is certainly right, 1) works for many >>DEC devices and 2) is sometimes needed. If you need 2), the most general >>case is as follows: You can just cross pins 2 and 3, then pins 4 and 5 are connected togeter on each side of the connector. Pins 6,8 and 20 are connected together on each side of the connector. This way you get the null modem cable you need. However you loose a little also, first no hardware flow control, second if you used this cable with a modem the computer would never see someone hang up the phone and the process attached to that modem would never die. A "generic" better may is illistrated below. Keep in mind RS232/DB25 may be a standrd, but many companys dont fully implement it or they add the own extra features. Generic NULL modem ------------------ 2__________3 3__________2 4__________5 !__6 5__________4 6__! 7__________7 8__________20 20_________8 You may or not need the pin 5 to pin 6 strap, If you are using AT&T terminals to and AT&T datakit LAN you will need the strap. What connectors go on each side depends on your enviorment, Typicaly a female on each end works out great, In a DEC only world all you need is female to female null modem cables. In an AT&T env. you might want to have male to female null modems. _______________________________________________________________________________ [ ] [ Gary W. Sanders (N8EMR) ] [ AT&T Bell Labs (Columbus,Ohio) ] [ 614-860-5965 (w) 614-457-4595 (h) ] [ 72277,1325 Compuserve. ] [ ] ------------------------------------------------------------------------------- -- test of sig file
hull@hao.UUCP (Howard Hull) (08/31/85)
I just perused the RS232 standard and the Application Notes and did not find the words "null modem" anywhere. I did find "DCE(modem)" mentioned once in the Application notes. So "null modem" must be trash slang. A "full" null modem can be quite complex. Following is a description for one I have had miscellaneous cable collection. DEC TERMINAL / COMPUTER RS-232 NULL-MODEM CABLE 10-MAR-82 Cable Type: Belden 9504 4 pair overall shielded Outside Dia.: 0.265 inches (6.75 millimeters) Connectors: DB-25 Terminal Computer Pin # Signal Name Wire Color Signal Name Pin # 1 Protective ground-------Shield Drain----Protective ground 1 2 Transmit Data---------> red ----------> Receive Data 3 3 Receive Data <--------- white <-------- Transmit Data 2 4 Request to Send--:----> violet -------> Carrier Detect 8 5 Clear to Send <--: 5 jumpered to 4 6 Data Set Ready <-:----- green <-------- Data Terminal Ready 20 22 Ring Indicator <-: 22 jumpered to 6 7 Signal Ground--white's blk & red's blk--Signal Ground 7 8 Carrier Detect <--- violet's blk <--:---Request to Send 4 5 jumpered to 4 :-> Clear to Send 5 20 Data Terminal Ready -> green's blk -:-> Data Set Ready 6 22 jumpered to 6 :-> Ring Indicator 22 Notes: This cable is wired like a DEC H312 Null Modem Card. Judging by the above note, at least DEC seems to think it knows what a "null modem" is. Howard Hull [If yet unproven concepts are outlawed in the range of discussion... ...Then only the deranged will discuss yet unproven concepts] {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull
jeq@laidbak.UUCP (Jonathan E. Quist) (09/01/85)
In article <1734@hao.UUCP> hull@hao.UUCP (Howard Hull) writes: >I just perused the RS232 standard and the Application Notes and did not find >the words "null modem" anywhere. I did find "DCE(modem)" mentioned once in >the Application notes. So "null modem" must be trash slang. A "full" null >modem can be quite complex. Following is a description for one I have had >miscellaneous cable collection. > >DEC TERMINAL / COMPUTER RS-232 NULL-MODEM CABLE 10-MAR-82 > >Cable Type: Belden 9504 4 pair overall shielded >Outside Dia.: 0.265 inches (6.75 millimeters) >Connectors: DB-25 >Terminal Computer >Pin # Signal Name Wire Color Signal Name Pin # > > 1 Protective ground-------Shield Drain----Protective ground 1 > 2 Transmit Data---------> red ----------> Receive Data 3 > 3 Receive Data <--------- white <-------- Transmit Data 2 > 4 Request to Send--:----> violet -------> Carrier Detect 8 > 5 Clear to Send <--: 5 jumpered to 4 > 6 Data Set Ready <-:----- green <-------- Data Terminal Ready 20 >22 Ring Indicator <-: 22 jumpered to 6 > 7 Signal Ground--white's blk & red's blk--Signal Ground 7 > 8 Carrier Detect <--- violet's blk <--:---Request to Send 4 > 5 jumpered to 4 :-> Clear to Send 5 >20 Data Terminal Ready -> green's blk -:-> Data Set Ready 6 > 22 jumpered to 6 :-> Ring Indicator 22 > >Notes: This cable is wired like a DEC H312 Null Modem Card. > >Judging by the above note, at least DEC seems to think it knows what a >"null modem" is. > Howard Hull DEC obviously hasn't used equipment that toggles Request to Send wired to a machine as above. Most all the UNIX terminal drivers I've seen would have a fit if the state of Carrier Detect was constantly changing. Wiring Data Terminal Ready on one end to Carrier Detect on the other end seems to nearly always work. (Data Set Ready can be substituted if equipment demands.) Request to Send should go to Clear to Send on the other end, and vice verse. If you're going to use it, it may as well be right. If equipment on one end doesn't support it, jumper them on *that* end. (A lot of equipment doesn't bother with them anyway; I've only seen it on printers that couldn't generate ^S/^Q (*gasp*) for flow control.) Safety Ground (pin 1) should NOT be carried all the way through, lest you develop ground loops. If you have shielded cable, by all means use the shield, though. More often than not, the best ground will be at the computer end. (This assumes a proper installation, all disks, power supplies, and cpu's grounding to a single point.) Jonathan E. Quist ihnp4!laidbak!jeq ``I deny this is a disclaimer.''
peter@graffiti.UUCP (Peter da Silva) (09/03/85)
Here is the minimum null-modem cable I have ever used. Note that it's really not suitable for anything but a hardwired terminal, short haul, but it's perfectly good for connecting DEC terminals to non-DEC computers & vv. 2------------3 3------------2 4-+ +-5 5-+ +-4 7------------7 6-+ +-6 8-+ +-8 20-+ +-20 Conscientious hackers will want to connect 1, 4 and 5: 1------------1 2------------3 3------------2 4------------5 5------------4 7------------7 6-+ +-6 8-+ +-8 20-+ +-20 But for most cases 6, 8, and 20 are irrelevant. Of course if for some weird fetishistic reason you want to look like a real pair of modems, leave 4 and 5 dangling & do something useful with 6, 8, and 20. The 3-wire cable at the top will handle a good percentage of your cabling needs.
hull@hao.UUCP (Howard Hull) (09/20/85)
> >Notes: This cable is wired like a DEC H312 Null Modem Card. > > > >Judging by the above note, at least DEC seems to think it knows what a > >"null modem" is. > > Howard Hull > DEC obviously hasn't used equipment that toggles Request to Send > wired to a machine as above. Most all the UNIX terminal drivers > I've seen would have a fit if the state of Carrier Detect > was constantly changing. Wiring Data Terminal Ready on one end > to Carrier Detect on the other end seems to nearly always work. > (Data Set Ready can be substituted if equipment demands.) The fact that it nearly always works is not an assurance that it complies with the RS232 standard. Will Martin was not trying to get something to work, he was concerned about what the definition was. I suppose that my submission of the DEC H312 Null Modem diagram is thus out of context; the H312 is an article of diagnostic and test hardware, and it is not used operationally. (However, I am sure the description of it was correct; there was a similar posting from Craig Bevins that came all the way from Australia!) I would like to resolve whether the either the software designers, or the hardware designers, or both, or neither are following the RS232 standard, and if not, what the violations are. The following is included for reference: Pin # Signal Name Description RS 232 Mnemonic 1 Protective ground (mutual) AA 2 Transmit Data (from T to M) BA 3 Receive Data (from M to T) BB 4 Request to Send (from T to M) CA 5 Clear to Send (from M to T) CB 6 Data Set Ready (from M to T) CC 7 Signal Ground (mutual) AB 8 Data Carrier Detect (from M to T) CF 20 Data Terminal Ready (from T to M) CD 22 Ring Indicator (from M to T) CE Notes: The "from" device asserts the signal. The "to" device reads the signal. Data Terminal "T" is the computer or a crt terminal. Data Set "M" is a modem, or a null-modem to a computer or crt terminal. You say: > Request to Send should go to Clear to Send on the other end, > and vice verse. If you're going to use it, it may as well > be right. The situation is complicated by the fact that the RS232 standard covers both dedicated line use and switched network use of modems (DTE) and terminals/host computers (DCE). There are handshaking protocols for each. For the purposes of this discussion, I will assume the switched telecommunication network is the use we are concerned about. From the Application Notes (Bulletin No. 9) of the EIA, I quote: --- 3.1.4 Handshaking A handshaking technique is used predominately on the switched telecommunication network. DCEs used on dedicated point-to-point circuits with alternate voice capability may also use this technique. The DCEs transmit signals to each other and perform certain timeouts to establish the data transmission channel and provide circuit assurance prior to turning the channel over to the DTE for data transmission. Where the DCE includes the handshaking function, it guarantees initial circuit assurance, and circuit CB (Clear to Send) (106) circuit CF (Received Line Signal Detector) (109) must mean exactly what the name implies. Each DCE has communicated with the other end and of the telecommunication channel and knows that it is ready to transmit and receive data. In the design of some DCEs this philosophy is carried even further in that the DCE will turn Circuit CB (Clear to Send) (106) OFF if the received line signal disappears from the telecommunication channel. This is based on the logical deduction, "If I can't hear him, he probably can't hear me either. Therefore, stop transmitting." --- Add to this some comments from Randolph Fritz UUCPnet: decvax!philabs!wu1!rf RANDOLPH FRITZ'S GUIDE TO RS-232 SIGNALS AND OTHER SICK JOKES. Signal DTE <-> DCE Description ====== =========== =========== 4 RTS -> Request to send. Turns on modem's transmit carrier. CONNECT. 5 CTS <- Clear to send. Indicates that modem's transmit carrier is on. Some modems assert this all the time. CONNECT. Message-ID: <274@wu1.UUCP> Date: Wed, 4-Apr-84 13:19:06 MST we then get a sequence in which as RTS is asserted by station A, its carrier must come on. That implies that it *was off*. The modem, having done this must then return the assurance that it has indeed done what it was told to do; the modem thus asserts CTS to the DCE it is connected to. There is no statement right here, expressed or implied, concerning what will happen at the other end of the telecommunication line. The two sources quoted above would indicate that DEC is correct in their alignment of Pins 4, 5, and 8, and that you are responding to a self-composed desire for a life that is simpler than the one the standard considers. It would appear that toggling RTS is only appropriate for dedicated lines, or perhaps for switched networks that remember paths, then proceed to connect and disconnect them with the abandon of a Tasmanian Devil in a field full of baby Jack Rabbits. Otherwise, one should either leave RTS alone or not drop carrier in response to null RTS. > If equipment on one end doesn't support it, > jumper them on *that* end. (A lot of equipment doesn't bother with > them anyway; I've only seen it on printers that couldn't > generate ^S/^Q (*gasp*) for flow control.) I know how you feel. I, too, would like the manufacturers to support both hardware and software handshaking. > Safety Ground (pin 1) should NOT be carried all the way through, > lest you develop ground loops. If you have shielded cable, > by all means use the shield, though. More often than not, > the best ground will be at the computer end. (This assumes > a proper installation, all disks, power supplies, and > cpu's grounding to a single point.) Safety Ground isn't a safety ground unless it IS carried all the way through, via a shield or conduit. What do you care if there is 5 Amperes of miscellaneous AC between two terminal frames, as long as the terminal innards are not connected to it? Deliberate execution of specified performance requirements must not be allowed to carry unsafe operational situations into the product system, even for wartime environments. For a system contained all in one building, and on one power distribution breaker branch, we would wire them as you do, with one end of the shield open, and the other grounded at the central distribution point, i.e. the computer. However, we do not have many installations that are all on one breaker branch. In those remaining cases (the majority), we take it tough and do it according to code. > Jonathan E. Quist > ihnp4!laidbak!jeq > ``I deny this is a disclaimer.'' Shucks. Me too. Howard Hull {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull