Sounds nice! Nothing wrong with using a 'shifter' and I'm just picking a nit, but the issue with interfacing RS232 to TTL in this case is not so much the voltage but the polarity. The ModelT outputs +5V to -5V and some TTL-type inputs don't like a negative voltage even if it's current limited as it is in the Model T's; it's recommended to clamp the -5V at 0V somehow, which your shifter takes care of. Receiving a TTL-level signal on the ModelT will usually work, but a bipolar RS232 is somewhat more reliable.
Still odd that you can't go above 1200bd; have you tried disabling XON/XOFF (with the loopbacks connected) in case the Arduino is sending an XOFF but never an XON? Since nothing changes on the T102 except the speed it would suggest that the other end can't keep up for some reason and a buffer is filling up somewhere, unless there's an issue with parity or framing. m ----- Original Message ----- From: "Ron Lauzon" <rlau...@gmail.com> To: <m...@bitchin100.com> Sent: Thursday, November 30, 2017 4:10 PM Subject: Re: [M100] Tandy 102 Text question > Well, the DTR/DSR loopback became moot. Both of the shifters that I > had that supported DSR/DTR failed. I couldn't get them to work at all > (even without the DTR/DSR settings). I think I messed them up a while > back. > > But it seems that 1200 BPS will work off the internal Mega serial > ports with the known good shifter that I have. > > So this project is a success. Thanks to all of you who helped me out. > I appreciate it. > > My project is documented on github > (https://github.com/rlauzon/MemoSaver) if you want to see what I did. > > It was a simple project, but it helped me pass the time until I start > my new job on Monday. > > > On Wed, Nov 29, 2017 at 6:02 PM, John R. Hogerhuis <jho...@pobox.com> wrote: >> >> >> On Wed, Nov 29, 2017 at 2:53 PM, Ron Lauzon <rlau...@gmail.com> wrote: >>> >>> >>> I broke out the RS-232 shifters that supports DSR/DTR and set them up >>> to loopback on the RS-232 side, but I couldn't get them to work. >>> I have a couple of things I need to check, but I think those shifters >>> simply don't work. >>> >> >> Hello Ron, >> >> Some confusion here, at least as to what I'm suggesting. >> >> DSR / DTR loopback is completely on the Model 100 side of the cabling. Has >> nothing to do with the level shifter or the Arduino. >> >> Imagine on the M100's DB25, directly wiring the M100's DTR pin to M100's DSR >> pin. That's it. So if you have a custom cable you're just shorting one pin >> to another at the DB25 end. No wire in the cable is involved, just at the >> connector. >> >> The old Club100 "comp-link" cable did this. It completely defeats the M100's >> DSR check, since whenever serial port is activated, the DTR is kept >> asserted. >> >> -- John. > > > > -- > Ron Lauzon - rlauzon at acm dot org > Homepage: http://webpages.charter.net/rlauzon/ > Weblog: http://ronsapartment.blogspot.com/ > > TRS-80 Pocket Computer 2 - TRS-80 Pocket Computer 4 - TRS-80 Model 100/102 > Some people like to work on old cars. But old computers are cheaper > and don't require a big garage.
