RS-232 flow control questions - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: February 18, 2018, 12:31:51 am
content: First, why the RTS/CTS/DTS/DSR? Why not just have a single signal on each side that goes high when it is able to receive and goes low when its buffer is full? What is the point of, are you ready, yes I am, etc. Are there systems that simply tell the other side if they can receive or not? rx (input), tx (output), rxReady (output), txReady (input)...

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 18, 2018, 02:30:04 am
content: Quote from: alank2 on February 18, 2018, 12:31:51 amBecause the protocol was developed when dinosaurs roamed the earth.

author: rstofer
author_rank: Super Contributor
timestamp: February 18, 2018, 02:40:02 am
content: The DSR/DTR signals supervised the phone line connection, I believe. The RTS/CTS supervised the flow of data.I go back to the days of acoustic couplers at 110 Baud and the phone connection wasn't always reliable.Mine was in a neat wood box with hinged lid. You dial the phone, wait for it to answer and then place the handset in the coupler. Yes, I still have a rotary phone. Somewhere along the way I lost my coupler...http://www.lm-technologies.com/wp-content/downloads/wireless%20adapters/LM048/Application%20Notes/DTR-DSR-RTS-CTS_Difference.pdf

author: rhb
author_rank: Super Contributor
timestamp: February 18, 2018, 03:52:08 am
content: RS-232 was used to connect almost everything. So everyone made sure their wires got included. I used to keep a kit of stuff in my desk at school to make up RS-232 cables. Fortunately I never had occasion to use anything but 2-3-7 for XON/XOFF. Printers and modems were where it got messy.My canonical test for a new sys admin was to say the word "modem". If they flinched I knew they had significant experience. I fixed a motorcycle for an admin at work in exchange for him configuring my Telebit Trailblazer. I fixed the bike no problem, but he never was able to make the Trailblazer work with SunOS 4.1 on a 3/60. Of course, before that I'd already spent days on it. He made a pile of money selling the ISP he started in his apartment, so he knew his stuff.Before we got Ethernet, I used RS-232 to connect a MicroVAX II in a BA123 "world box" to the 11/780 4 floors down. We ran the wires in the HVAC plenum, so I had to buy special wire.

author: ikrase
author_rank: Regular Contributor
timestamp: February 18, 2018, 05:33:25 am
content: RS-232 was originally transmitted over DB-25 connectors, which really contained the kitchen sink -- including not just one but *two* UARTs.So there were a fair number of things available... so they got used... and ported into the DB-9 that is far more familiar as a serial connector.Of course, it's definitely useful to have *some* pin that you can use to toggle the RS-485 transciever. I wish we had RS-485 as the standard rather than 232.Of course, all those pins came to some use. In the early 2000s you could buy serial PIC programmers that used the flow control pins to bitbang the PIC synchronous serial ICSP protocol. Meanwhile, a continuous stream of 1's and 0's at high bandwidth on the data line would drive a simple charge pump to provide the high (12V+) programming voltage needed for older FLASH PICs.

author: drussell
author_rank: Super Contributor
timestamp: February 18, 2018, 04:19:31 pm
content: Quote from: alank2 on February 18, 2018, 12:31:51 amSimple. Because RS-232 is intended to connect a terminal to a modem and a modem to a computer and you need to know more than just "you can send".DataTerminalReady is needed for the terminal to tell the modem it is ready. Drop this and the modem hangs up.DataSetReady means the modem is ready to send / receive data.CarrierDetect tells the terminal that the modem has a connection to another modem.RequestToSend tells the modem you want to send data out from your terminal.ClearToSend tells the terminal that the modem is ready for sending data.etc. etc. This is all well documented and comes from the historical needs of the interface in its intended usage. i.e.:QuoteRTS/CTS weren't originally intended for "flow control" from buffered modems like we think of in "modern" times. It was from very early half-duplex operation but was used for the similar property of "flow control" later when effective line speeds started to not be == local terminal connection speed due to compression with MNP, etc. etc.

author: glarsson
author_rank: Frequent Contributor
timestamp: February 18, 2018, 06:51:22 pm
content: Quote from: ikrase on February 18, 2018, 05:33:25 amDE-9 please. A DB-9 would be a nine pin connector in the same shell as a DB-25.D is the connector shape/type D-subminiature.Second letter is the shell size.The number is the number of pins.DB-25 Big serial.DE-9 Small serial.DE-15 VGA.DD-50 Old SCSI connector.DB-13W3 Size as big serial but with 3 coax and 13 normal pins (was common on Sun workstations).