modular serial hardware (was Re: a printer oddity)

On Fri, Apr 11, 2008 at 05:04:12PM -0400, Roy J. Tellason wrote: We were using RJ-11-based serial gear from Nevada Western in the mid-1980s. I still have it, and have a bunch in use (a 25-pair telco cable going from a wall box in my computer room with 8 RJ-11s to a 19" panel with groups of RJ-11s mounted in the space behind my VAXen in the basement). We had two successive buildings wired with the stuff - we made our own pin swabbers to take Emulex CS21 cables and fit them into the pattern needed by those telco adapter blocks, and filled an entire relay rack with the 19" panels... to add a new interface, we'd throw the card in a VAX or PDP-11, cable it up to the back of the relay rack, then install RJ-11 patch cables to route TX0 or TTA0 or whatever to whatever room we wanted to login from. Very, very handy. You might check the pins before randomly rewiring them... For RJ-45s, the most common arrangement I've seen is the one presently used by Cisco - the center two conductors on the cable are both ground, then flanked by TxD and RxD, then flanked by handshaking lines. Most of the gear I've seen lately that uses RJ-45s for serial, though, stops with the inner 4 pins. One handy thing about this interconnection scheme is that you don't _need_ to change the shells for null modems (though you can). You can flip one end of an RJ-45 cable (making it unsuitable for Ethernet use) to make the cable a null modem - the signals are arranged in a "paired" fashion to make that work. In practice, the flat cables that Cisco ships are assembled inverted compared to "Ethernet" cables, so it matters what you grab when you link up two devices. OTOH, I rarely use the RJ-45 scheme with DB25 connections, since the shells are rarer, in my experience, compared to DE9 shells (which are *quite* common in the Cisco world). At home, for DB25 stuff, I tend to use my boxes and boxes of RJ-11 stuff, unless I have a connection that needs "proper" hardware handshaking - then I usually bust out a "real" cable. -ethan -- Ethan Dicks, A-333-S Current South Pole Weather at 12-Apr-2008 at 03:20 Z South Pole Station PSC 468 Box 400 Temp -61.4 F (-51.9 C) Windchill -95.8 F (-71.0 C) APO AP 96598 Wind 11.2 kts Grid 34 Barometer 679.2 mb (10658 ft) Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html