SWTPC 6800

On Fri, 5 Aug 2016, Brad H wrote: OK, I don't know what your level of knowledge is on it. Mine is NOT very high, so please don't be offended if I tell grandma how to suck eggs. <over-simplified> I would have hoped that, expecting a terminal, it would be wired as DCE (Data Communications Equipment), and plug straight through to a DTE (Data Terminal Equipment). However, a PC is NOT expecting a terminal to be connected to it, and is wired DTE! (PC is a terminal expecting to connect to a modem, not something to connect a terminal to.) Therefore, ideally, the PC should connect straight through, with only software issues of baud-rate, parity, bits, stop bits, and software handshaking. That obviously is not the case, and rarely is. Oh, wouldn't it be great if it worked that easily. A null modem is for connecting two DTE terminals to each other. It connects the transmit of one side to the receive of the other, and often numerous hand-shaking signals, which are NOT really standardized. (RTS,CTS,DSR,DTR,CD) For example, if you want to connect a PC (DTE) to a printing terminal to use that as a printer, you need to cross TX and RX, and then mess with handshaking enough to get the PC to send characters when the printer is ready, and stop sending when the printer is not ready. Sometimes, that is done by connecting handshake lines so that the PC ALWAYS sees the printer as ready, and then just send the characters slowly enough that the printer can keep up. (typically 300 baud for daisy wheel, 150 or less for selectric). Sometimes stop bits can help, if things are close. They are usually wired symmetrically, but that doesn't always work. With full handshaking, that crosses TX and RX DTR and DSR RTS and CTS With partial handshaking, that may be TX and RX RTS and CTS DTR and CD+DSR sometimes: TX and RX DTR and DSR RTS and CTS+CD It is kinda essential to get or make a breakout box. 1) to be able to try different interconnects 2) to have LEDs showing you the state of the lines. Joe Campbell's "The RS232 Solution" is at a level that I can follow. 'course, there's always LUCK. I had an assistant, who, the very first time that he saw a serial printer, he reached into one of my bins of random cables; and it worked! </over-simplification>