Some of the original M100s have the wrong resistors in part of the RS232 handshake circuit. The voltage drop is so high that with the signal looped back it is just barely sufficient. The Backpack eavesdrops to know when to power itself back up and this minuscule additional load is too much. The symptom is that the Backpack responds once and then stops.
I have tried to attach a tech bulletin that Tandy put out back in the day that describes the fix but it was too large. I zipped it up with the M200 andM600 tech bulletins here: https://1drv.ms/u/s!AtH4vpaZnzX7lq9GsKp0I4qvY59yCw?e=yAcrwQ The alternative is to set both lookback switches away from the dot. The Backpack does not eavesdrop in this configuration so it can't wake itself up. You can adjust the timeout to compensate. This is only the third M100 I've com across with this issue. I have one of them too 😊 At some point in the future I'll do a video on the fix per the Tandy tech bulletin. Jeff Birt -----Original Message----- From: M100 <[email protected]> On Behalf Of Philippe Van Lieu Sent: Friday, January 13, 2023 2:02 PM To: [email protected] Subject: [M100] Backpack-Plus with REXCPM help Hello everyone! I recently bought the Backpack-Plus for the M100, and I LOVE the thing. I want to thank you Jeff Birt/soigeneris for making this wonderful device to help keep these classic machines alive! That said, I'm having an issue getting it to work properly on my REXCPM-powered M100. Fortunately the Backpack works fine on my normal M100 so I figure the Backpack isn't the issue, but every time I try to run it on TS-DOS on my REXCPM'd M100, I get a "device not ready" error albeit after working only once beforehand. I read the instructions and tried to adjust the timeout/sleep settings to something like 5 minutes via the CLI, but that didn't affect it. I also read the "Use with CP/M-100 and REXCPM", but that's just the initial installation instructions. I also saw the suggestion of using TS-DOS v4 or higher, but that also didn't have a positive effect. Finally, a saw the paragraph on page 45 about making sure that the LOOP switch is set to LOOP in order to ensure TS-DOS keeps the Backpack-Plus awake, but neither that nor power cycling the Backpack-Plus woke it up. So I'm at a loss on what I can do to get it working. I went through the M100 mailing list archive, and it sounds like other REXCPM users haven't had any issues using the Backpack... so I suspect that maybe I'm doing something wrong. I'm open to any suggestions, and I thank you ahead of time for your help! --- Philippe
