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28 Making a Null Modem -=> How <=- 18 Open ED.BELL [* sysop *] ************ Topic 28 Mon May 21, 1990 ED.BELL [* sysop *] at 18:39 EDT Sub: Making a Null Modem -=> How <=- With the new UART cartridges and the promise of obscene transfer speeds using null modems, I would like to get some information on putting one of these things together. Any one have the information??? 18 new messages. ************ ------------ Category 8, Topic 28 Message 1 Mon May 21, 1990 ED.BELL [* sysop *] at 18:43 EDT I need some detailed information quick on the construction of a null modem. I want it to connect my C64 w/my C64C and my C128. Answering etc. are not important... just being able to communicate across the line. Can anyone help me with some pointers or a whole outline, esp. with part numbers and sources and appr. cost. Thanks much. I think this is going to be a frequently asked question with these new UART cartridges now hitting the market. ------------ Category 8, Topic 28 Message 2 Mon May 21, 1990 DIGITAL.DOC at 22:37 PDT Making a null modem cable is simple....Merely a matter of cross- connecting appropriate control lines ------------ Category 8, Topic 28 Message 3 Tue May 22, 1990 ED.BELL [* sysop *] at 12:19 EDT Which lines DOC? This is kind of new, at least this end of it. I'm used to opening the box and plugging 'em in to the phone and computer and just struggling to program them. ------------ Category 8, Topic 28 Message 4 Thu May 24, 1990 DIGITAL.DOC at 20:50 PDT Back with that info next week!! :) ------------ Category 8, Topic 28 Message 5 Fri May 25, 1990 C128-BILL at 08:58 EDT Ed, I made up a few a while back. Just purchase a 25 wire ribbon cable and two male RS232 snap in connectors from R/S. Before you snap the connectors on, swap wire #2 and #3 on one end (#1 has the red stripe). Try and keep the ribbon cable short, and away from other electrical devices. The whole set-up costs about 10 to 15 bucks, and takes a few minutes to do. Use shielded cable for serious work, but then you need to drag out an iron. Radio SHack also sells a Null Modem plug for about the same price, with pins 2 & 3 already swapped. One end is a female, the other a male. The real trick is to have some sort of switching box so you can flip one or two switches and have both computers talk to each other, or either one use the same modem. Generally one or two 232 switch boxes will do, costs about 15-20 bucks each, but it saves you the hassle of unplugging/plugging cables. Having two COM ports one one 'puter is handy for this, but I don't know if the Amiga has this feature. Even with one COM port a max of two switch boxes can be made to 'do' the above switching, but you have to be a little 'creative' with your wiring. :) ------------ Category 8, Topic 28 Message 6 Fri May 25, 1990 ED.BELL [* sysop *] at 09:49 EDT Sounds great Bill (and DOC). I do have one little curve in the equation though.... the cartridge goes into the C64 cartridge port and has a db9 (joystick)) port. There is a cable that comes with it that goes from the db9 to a db25. Guess that doesn't really change it much huh? I'm glad all this stuff is on the books here tho so I can go back and access it any time I'm ready. And any more detail is also greatly appreciated. Not lazy, just *BBBUUUUSSSSYYYY* Oh, the reason I brought up the cart ports is that the cable from the cart plugs directly into the modem, so apparently does the work of an interface, so I just don't want to do anything to hurt either the cartridge or my computers. Is this a valid concern DOC. It has been my understanding that the interfaces have something to do with the power from the computer as opposed to what is used in the standard world. ------------ Category 8, Topic 28 Message 7 Fri May 25, 1990 DIGITAL.DOC at 22:00 PDT Doing a little investigating...will be back this weekend with an answer. ------------ Category 8, Topic 28 Message 8 Sat May 26, 1990 WC.COLEMAN [*Sysop*] at 03:36 EDT The computer uses +/- 5 volts DC while standard RS232 uses (if I remember correctly) +/- 12 VDC. All the interface does is convert the voltages over. BTW if you are connecting to a printer you will probably also have to swap the DSR and DTR lines. It's been ages since I've done this stuff but I'll see what I can dig up. -WC ------------ Category 8, Topic 28 Message 9 Sat May 26, 1990 ED.BELL [* sysop *] at 08:42 EDT I didn't remember the particulars, but I knew it was something to do with the voltages. I have no interest in making it for a printer... just for the UART cartridge. Since this thing has a cable ending in a db25 cable connector for the modem, it apparently serves as its own interface and I was concerned about reconverting a converted signal. I have been thinking that I could get a double female db25 connector to just link my interface to the cable from the cartridge and have a null modem. Does that sound like sound logic? ------------ Category 8, Topic 28 Message 10 Sat May 26, 1990 R.RANDALL5 [Zeroy] at 10:59 EDT A little thought would reveal that your ribbon cable recipe could not possibly work. While it is true that you would want to swap PINS two and three to convert a modem cable to a nul-modem cable, that is not the only swap that needs to be done, AND conductors two and three in a ribbon cable do not correspond to PINS two and three in the DB25 connector. Since the DB25 has two rows of pins, every other ribbon conductor goes to a different row. So if the red conductor is pin one, then the adjacent conductor is pin 14 and the NEXT one is pin 2. There is no standard for a nul-modem cable because different interfaces ignore or use different pins. The very simplest cable would cross pins 2 & 3 with pin 7 straight through. It would also be advisable to cross pins 6 and 20. From here it can get kind of sticky. If one of the devices is a PC you should loop pin 4 back to pin 5 on one end. In fact, it might be a good idea to short pins 4, 5, & 8 on both ends of the cable. I'm not trying to cloud the issue. The point is that different interfaces using different software may or may not pay attention to different pins. ------------ Category 8, Topic 28 Message 11 Sat May 26, 1990 ED.BELL [* sysop *] at 14:59 EDT Huh? Thanks for all that... now all I have to do is find some quality time to digest it. :-) Actually, the reason this is such a sticky thing is that the modem interface from the cartridge does not use the user port (where the modems normally go, in case I used the wrong name). It uses the port where the cartridges go, and has a 9 pin connector to which a cable connects and makes the link to the modem. I am no expert on these null modems in the first place, and I have an added dimension to the problem. I just hope when I'm ready to do it, the information presented here (and which I hope continues) is enough, and understandable enough, for me and anyone else interested to make one. ------------ Category 8, Topic 28 Message 12 Tue May 29, 1990 C128-BILL at 18:22 EDT Ah! Thanx Roy for pointing that out. Pins 2/3, not wires 2/3. However in msg 10, Roy, you mention shorting pins 4 & 5. I haven't seen that one, yet. The one I have for here has 4/5 crossed (swapped) and not and not tied to 8. The 6/20 might have to be swapped, too (DSR). Thanx for the correction. :) ------------ Category 8, Topic 28 Message 13 Sat Jun 02, 1990 R.RANDALL5 [Zeroy] at 00:51 EDT I told you this might get sticky. Properly, 4 & 5 should be crossed. However, some devices don't use 4 & 5 while others depend on them. PC serial ports usually want to see pin 5 high. If a the attached device (say, a Commodore) doesn't put pin 4 high, then the PC will refuse to send to it. Therefore, we can fool them both by looping back their OWN pin 4 to their own pin 5. Pin 8 is Carrier Detect. Again, some terminal programs don't pay any attention to CD. Some modems will hold pin 8 high all the time. Some modems hold pin 8 high ONLY if it is properly synched with another modem's carrier. My C=64's serial port seems to ignore most of this stuff. The question then remains, what are you hooking it to, and what does it expect to see? ------------ Category 8, Topic 28 Message 14 Wed Jul 04, 1990 DIGITAL.DOC at 20:17 PDT ED, As promised a few decades ago, here's the pin-out and interconnect for RS- 232C. Here is the standard RS-232C pin-out for the DB-25 connector: Pin Name Function 1 AA Chassis Ground/Common 2 BA Data from terminal 3 BB Data received from Modem 4 CA Request to Send 5 CB Clear to Send 6 CC Data Set Ready 7 AB Signal Ground 8 CF Carrier Detection 9 Not Used 10 Not Used 11 Not Used 12 Not Used 13 Not Used 14 Not Used 15 DB Transmitted Bit Clock (Int) 16 Not Used 17 DD Received Clock Bit 18 Not Used 19 Not Used 20 CD Data Terminal Ready 21 Not Used 22 CE Ring Indicator 23 Not Used 24 DA Transmitted Bit Clock (Ext) 25 Not Used The "Standard" Null Modem cable is made as follows: Connector 1 Connector 2 ----------------------------- Pin 1 Pin 1 ----------------------------- Pin 2 Pin 3 ----------------------------- Pin 3 Pin 2 ----------------------------- Pin 4 Pin 8 Pin 5 ----------------------------- Pin 6 Pin 20 ----------------------------- Pin 7 Pin 7 ----------------------------- Pin 8 Pin 4 pin 5 ----------------------------- Pin 20 Pin 6 ----------------------------- There ya go. Be aware that different equipments do not always use the same pin to signal configuration. <Doc> ------------ Category 8, Topic 28 Message 15 Thu Jul 05, 1990 ED.BELL [* sysop *] at 01:40 EDT Excellent DOC! And, as much of the technical stuff you put out here, this would make an excellent tidbit for the SBJ. pssst. Sheri Lynn... ------------ Category 8, Topic 28 Message 16 Thu Jul 05, 1990 DIGITAL.DOC at 18:19 PDT <Blushing!!> ------------ Category 8, Topic 28 Message 17 Fri Jul 06, 1990 ED.BELL [* sysop *] at 03:25 EDT No need for that DOC! I looked at the file on hard copy and it is even better than I originally thought. I think this should be in the next SBJ! ------------ Category 8, Topic 28 Message 18 Sun Jul 08, 1990 DIGITAL.DOC at 01:04 PDT Thanks Ed! Just doing my thing, as it were! ------------