binder@dosadi.DEC (Wherever you go, there you are.) (04/09/85)
It's generally considered poor form to connect pin 1 of a DB25 cable when making a null modem. Pin 1 is the SHIELD ground, and is sometimes connected to mains ground in the terminal or whatever. You can get serious ground loops by hooking up pin 1. Cheers, Dick Binder (The Stainless Stee Rat)
rpw3@redwood.UUCP (Rob Warnock) (04/14/85)
+--------------- | It's generally considered poor form to connect pin 1 of a DB25 cable when | making a null modem. Pin 1 is the SHIELD ground, and is sometimes connected | to mains ground in the terminal or whatever. You can get serious ground | loops by hooking up pin 1. | Dick Binder (The Stainless Stee Rat) +--------------- No, no, no! It's poor form NOT to connect pin 1 !!! Yes, pin 1 is the shield, but if you don't connect it, you have lost shield continuity and will start radiating (and sometimes receiving) all kinds of crap. If there are ground loop problems, it will be because one of the terminals didn't handle pin 1 correctly internally. Note that pin 1 and pin 7 are always connected at SOME point, unless "logic ground" is "floating" (a VERY bad practice!). But you want pin 1 to take the burden of any ground-loop current, precisely to keep it OUT of pin 7. Pin 7 usually is connected to the logic board ground near the RS-232 drivers and receivers, while pin 1 should be connected only at the single common point of frame, power, and logic ground (usually physically near the power mains inlet wire or power cord). If you don't connect pin 1, any ground- loop currents WILL flow through pin 7, and through the logic boards on one or both ends. This can be very upsetting to the poor logic. Another way to look at it: Both Pins 1 & 7 should theoretically be at zero potential, but if they are not, pin 1 (the shield) acts as a "guard" for pin 7 (logic ground). Final point: If the shield is broken in the middle (such as a null modem with no pin 1 connection), the entire cable length becomes an antenna, both broadcasting and receiving. Depending on where your Apple and your Kaypro are, that could cause FCC problems (if your neighbors complain) or data errors (if the cable runs up, say, an elevator shaft). Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404
cdh@BBNCD3.ARPA (Carl D. Howe) (04/16/85)
There is often much confusion on the role of pin 1 in RS-232. As it turns out, both Dick Binder and Rob Warnock are right. It is the responsibility of the DTE (read terminal or computer) in a system to connect pin 1 of an RS-232 cable to frame ground to prevent radiation from the cable, provide shielding, etc. However, the DCE (read modem) DOES NOT connect pin 1 to frame ground to avoid ground loops between the DTE and the DCE. A null modem simulates two DCE's back to back. So when you are making a null modem, it is best to leave pin 1 open on both sides; the DTE's have already tied pin 1 to their frame grounds so the cable's won't radiate, and your leaving them separate from each other prevents ground loops. Carl