The Backpack has two modes of operation: CLI and TSDOS Server. Since it is powered by a single AA battery in order to conserve power it spends most of its time sleeping unless it is asked to do something.
In CLI mode it communicates with a terminal program on the host computer. This makes it easy to set time/date, mode of operation, or even transfer files via XModem. If it goes to sleep you can wake it up with 4 carriage returns in a row. In TSDOS server mode it has to be a little smarter. TSDOS asserts DTR and expects to then find DSR set. To save power, so the Backpack does not need to stay powered on to provide the DSR signal you can set it to loop back the DTR signal from the computer, i.e. DTR to DSR. The Backpack is fast enough to wake up and handle requests from the TSDOS client on the computer by monitoring this looped back signal. It can remain in peaceful slumber using very little power until TSDOS wakes it up. On the M100 there is a large resistor in the DTR output path AND a large resistor in the DSR input path. If you loop DRT to DSR the signal seen at the level shifting chip in the M100 is just over 1V (from my memory.) If the Backpack is also eavesdropping on this looped back signal the voltage goes below 1V and the M100 no longer sees the DSR signal. According to the tech bulletin this was also a problem when trying to use the M100 with a few of Tandy’s own products at the time. If you turn off the loopback switch for DSR/DTR the Backpack will have to generate DSR thus it needs to stay powered up. A work around is to turn the lookback off and then increase the sleep time to 2-5 minutes. Replacing the resistors is not difficult but it is a task that some folks don’t wish to do themselves, so the workaround is a good option. I know of someone who has an NEC 8201A with this same issue though I have not dug into it in detail to see if they had updated this section of the circuitry. Jeff Birt From: M100 <[email protected]> On Behalf Of John R. Hogerhuis Sent: Friday, January 13, 2023 4:04 PM To: [email protected] Subject: Re: [M100] Backpack-Plus with REXCPM help Interesting. RS232 has always been marginal on the Model 100 but it's news to me that there was a needed repair. I'm also interested to understand more about what's going on with the backpack. You said it had to do with handshaking signalling and loopback, and what sounds like a powersave mode in the backpack reacting to the low signal. What is "loop" feature on Backpack? Why is it necessary? Does Backpack not connect or assert DSR? -- John.
