newrace@hogpc.UUCP (K.LUKAS) (10/20/83)
Wanted: Info on minimum EIA RS 232-C leads needed on host
end of link. Specifically - is RING INDICATER needed on that
interface? What are the consequences of it not being supported?
Please mail responses to me - I'll summarize to the net.
Thanks in advance.
karel lukas { inhp4!houxm! or pegasus! } hogpc!newrace
or hogpc!kliosmith@umn-cs.UUCP (10/23/83)
#R:hogpc:-27000:umn-cs:1600002:000:1026 umn-cs!smith Oct 21 13:01:00 1983 The 'absolute minimum RS-232C' I've ever seen is three wires: transmit, receive, and signal ground. Will this work for you? No way to tell without knowing what you're hooking together. Here are observations based on my own experiences: 1. Everyone expects at least transmit, receive, and signal ground. 2. I've seen few systems that really require a RING signal. 3. Many (but not all) connections use Data Terminal Ready (DTR) or Data Set Ready (DSR) to tell if there's something connected. Dropping one of these (depending on your direction) "disables" the connection. 4. Carrier Detect is usually important if you're connecting to a modem. 5. Some people use Request to Send/Clear to Send (RTS/CTS) instead of or in addition to DTR/DSR. 6. Any of these signals can usually be strapped permanently 'on' when you build the interconnecting cables. This takes away your control but can make otherwise impossible things work. Rick Smith. U. Minnesota [smith.umn-cs@Rand-Relay] [...ihnp4!umn-cs!smith]
ptw@vaxine.UUCP (P. T. Withington) (10/25/83)
This is the closest thing I've ever seen to a "universal" RS-232 cable:
(Computer End) (Terminal End)
Shield 1-----------------------------------------------1
XMT 2---------------------\ /-----------------------2
X <=======================(makes a null modem)
RCV 3---------------------/ \-----------------------3
RTS 4----+ +----4
| |
CTS 5----+ +----5
DSR 6----+ +----6
GND 7----)-------------------------------------)----7
CD 8----o----------------\ /..................o----8
X
DTR 20...................../ \----------------------20
If you leave out the "..." line, short 8 to 20 at the terminal end (losing the
the ability of your terminal to tell if your computer is there, which few do
anyways). This cable can thus be implemented with a four-wire shielded wire,
or phone wire if you don't believe in shielding (which is sometimes a loss).
Someone out there is marketing an "automatic" black box, which purports to
instantly connect two RS-232 devices. I've ordered one, but haven't seen it,
and don't remember off hand who it is.
't` --Tucker (ptw@vaxine.UUCP)
~mason@utcsrgv.UUCP (Dave Mason) (10/26/83)
The absolute minimum number of leads is 2! I have a friend with a
timesharing company, and he uses 2 wire cables for the VT100s around
the office: pins 2 & 3 and depends on the floating ground from the
power supplies to be within 3 volts or so (a safe assumption within
one building). Never ceases to amaze me that it works fine.
-- Dave Mason, U. Toronto CSRG,
{cornell,watmath,ihnp4,floyd,allegra,utzoo,uw-beaver}!utcsrgv!mason
or {decvax,linus,research}!utzoo!utcsrgv!mason (UUCP)dag@fluke.UUCP (David Gunderson) (10/31/83)
Be careful about floating ground systems. A fellow I know connected up an RS-232 system like that in a building remote from the one the computer was in. He used a direct wire connection. It turned out that the outlet his terminal was plugged into had the ground and common reversed causing the signal lines of the RS-232 line to be raised to about 110 volts. It caused MUCH damage in the serial interface section of the PDP-11 that it was connected (smoke and flame!!!). --David Gunderson
karn@eagle.UUCP (Phil Karn) (11/02/83)
For those who need to handle dissimilar grounds between terminal and computer, I noticed that Inmac (that company that must spend half its profits on sending me catalogs every week) is now selling RS-232 fiber links. You might consider this to be a rather stretched opto-isolator. This isn't an endorsement of Inmac; theirs just happened to be the first ad I've seen for this kind of product. Phil
david@varian.UUCP (11/03/83)
The "automatic black box" to connect 2 RS232 devices referred to by
vaxine!ptw is called the SmartCable from IQ Technologies, 1181 N.E. 1st
St, Bellevue Wa. 98005 (206) 451-0232 (that's a cute phone # :-)).
Call them to find your local distributor.
We received one a few weeks and found that it really works in most cases
(we found a few exceptions involving HP equipment that I don't remember
off hand). I haven't taken it apart yet to examine the innards, but
it seems to work by overkill - it turns on all possible control signals
in the hope that it will hit the ones you need. It is also capable
of being a null modem (reversing pins 2 and 3) and seems to work fine
as such. It has already saved me quite a bit of time in making quick
connections, where in the past I would have had to play with a breakout
box and jumpers.
The cost of the logic module (SC817) is $62 (there is another version
called SC817-D that contains one extra LED for debugging - it's about
$15 more and I would have ordered that instead if I had known) and consists
of a male 25 pin plug, ribbon cable connecter, 2 LED's, and a switch
(you flip the switch til both LED's are lit). For another $15 I got
the 257-Dual cable (ribbon connector to both male and female 25 pin
plugs).
David Brown
Varian Instruments 2700 Mitchell Dr. Walnut Creek, Ca. 94598
(415) 945-2199
{ihnp4,tektronix,sytek,dual}!zehntel!varian!david
{amd70,fortune}!varian!david
...!decvax!sytek!zehntel!varian!david
...!ucbvax!menlo70!sytek!zehntel!varian!davidsmith@umn-cs.UUCP (11/05/83)
#R:utcsrgv:-257700:umn-cs:1600003:000:450 umn-cs!smith Oct 29 10:40:00 1983 Concerning 'floating grounds': I remember a story from a few years back: a NY bank was installing a vast number of terminals and associated datacom hardware. Nothing worked. Finally a smart EE checked out the building power and discovered that the typical office area had power coming in from two DIFFERENT mains and there was a significant voltage drop across the alleged 'grounds'.... Rick. [smith.umn-cs@Rand-Relay] [...ihnp4!umn-cs!smith]