On Wed, 2008-07-09 at 00:57 -0400, Gene Heskett wrote: > On Wednesday 09 July 2008, Les wrote: > >Gene and others will correct whatever I get wrong... > > > > The original specification was for a serial interface to multiple > >devices. If you look at the entries below 0x20 on the ASCII table, you > >can gather such gems as DC1 and SEND and some others. These were meant > >to control multiple devices on a single serial port. This is part of > >the software specification. The hardware specification implemented two > >serial busses in a single connector. Although most hardware never > >implemented the second bus, instead treating the one as bidirectional, > >which reduced the effectiveness of RS232. Also most software folks > >outside of IBM and UNIVAC seldom implemented the full control spectrum. > > > > The line drivers were supposed to be cabable of 20ma loop > >current. This meant that the serial loop could pass through a chain of > >devices, each of which could receive and transmit data. > >You could select which device(s) were active by chosing device control > >codes which would act on specific hardware in the receiving device. Its > >primary use was for communications, driving data to modems and receiving > >data from modems. > > > > Generally most folks would ground the shield, which was sort of an > >error. What should happen is that the shield is the return path for the > >transmitted signal. from the DTE. Signal ground is the return path for > >the signal originating at the DCE. Thus properly implemented the two > >sides have different ground references. At the DCE (modem or printer > >for example) the signal ground carries the return path for the signals > >going to the DTE. the shield carries the signal return to the DCE. > >Thid implied that who ever was working on the line had to remember which > >line he was on. 200' of line in a high RF environment or high loss > >environment could build up quite a charge if it were not dealt with > >properly. Inside the equipment, the receiving block at the DCE would > >tie its input circuits between shield and a floating 5v signal. It was > >never really floating, as there would be a high value resistor between > >the shield and signal ground. The transmitting block of the DCE would > >be tied between DCE +5 and signal ground. > > > > At the DTE end its transmitting block would be tied to +5 and shield > >to ground, with signal ground connected by a high value resistor. Its > >receiving block would be between a floating 5V and signal ground. > > > > NOW all that is the theory. In practice shield ended up being > >connected to case grounds on both ends, and signal ground was used as > >the reference. This meant that you had some signal degradation on > >simultaneous transmit and receive, resulting in the buss being used > >alternately for send and receive. Also most manufacturers didn't want > >floating supplies, so that ended. Finally they balked at the additional > >$3.00 or so for the line driver bits, and used normal 7400 or DTL logic > >for the lines. the result was lower speeds, shorter distances, loss of > >noise immunity, and too many unused pins on the connector. In came the > >DB9, and a host of other hardware interfaces, out went the lines > >available for ringing (the anonymous test lines), and so went the story > >of slow, limited line length RS232. > > > > So basically all you need is the pinout in the bottom connector of > >the drawing (DB-9) or one of the other interfaces, short leads > >(generally <6') and you should be all set. There isn't much that can go > >wrong other than you may need a null modem connector to transpose the > >two serial wires. They are available all over the place, or you can > >simply open up the offending connector and swap them there. > > > >Regards, > >Les H > > > > The original interface was a 25 pin device: > >from: http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html > > And that is a better description than I could ever have written. I wasn't aware > that some of the odd signals were actually a second data channel. I also do > not think any of that is covered in "Understanding Serial Communications" or > whatever it was that the Shack was selling as The Bible of rs-232 20 years ago, > and which I have a copy of lolling around here someplace. > > Unforch, the hardware side is only half of the story, the handshaking to assure > good data is the other half, and that can get fairly complex too as it appears > there is more than 1 interpretation of the '7-wire' protocol now so widely used > with the db9 connector cuz even that leaves 1 empty pin. The RI is rarely > connected or used, although some modems may have exerted it back in the dim > reaches of time 2 decades ago. > > -- > Cheers, Gene > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > Fishbowl, n.: > A glass-enclosed isolation cell where newly promoted managers are > kept for observation. > Thanks guys, This has jogged the memory a bit. A friend has a book by Scott Mueller that has a bit on serial I/O. With all this and google I should be able to get started. That rainy day is coming... -- Hooroo, Simon -- fedora-list mailing list fedora-list@xxxxxxxxxx To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
