TheBackShed.com

Home	JAQForum Ver 24.01
Log In or Join
Active Topics
Local Time 17:09 11 May 2025	Privacy Policy
Jump to

Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.

Forum Index : Microcontroller and PC projects : Micromite / VT100 comms. oddity

Author

Message

jsand
Newbie

Joined: 02/07/2014
Location: South Africa
Posts: 15

Posted: 10:16pm 02 Jul 2014

Copy link to clipboard

Print this post

Hello BackShed,

I've just got my PIC32 going with the Micromite 28pin hex loaded
and the led blinker program working, so I'm proud that the platform
is up and running.

Setup is:
IBM Notebook PC with a real serial comms port (COM1).
Running Tera Term as console.
Micromite flashed using MPLAB, Microstick II etc.
PIC32MX250F128B.

On inital power-up all I got was garbage on the console, but at
least it was garbage instead of nothing. Evidently the Micromite was alive.
Inspection showed the Rx/Tx lines looked to be running inverted.
So, I hooked in a CMOS 4xxx to invert them again & the whole
thing ran sweetly.

My question: Why the need to invert?

rgds, John

Grogster

Admin Group

Joined: 31/12/2012
Location: New Zealand
Posts: 9489

Posted: 10:31pm 02 Jul 2014

Copy link to clipboard

Print this post

Very common thing, and nearly always needed. Page 57 & 58 of the MicroMite manual describe a low-cost 232 interface though. You still need a level-corrector for the console though.

TTL is 5v(or 3v3) logic, but RS232 is 15v logic.

I'm not sure why exactly, but RS232 being what it is, is simply not compatible at a voltage level end of things, and no idea really, why they decided to make RS232 inverted(idle-low), and TTL true-logic(idle-high). There WILL be a reason, but I am not sure what that is - others will probably chime in with reasons.

As long as I have EVER done ANYTHING with com ports, I have ALWAYS needed a level corrector like a MAX232 in-between any 5v(or 3v3) logic, and a PC - C'est La Vie...

Oh, and welcome to the forums, and congratulations on you working out what the issue was - well done you, and I now hope you get many happy hours of experimentation from your MicroMite.

Edited by Grogster 2014-07-04

Smoke makes things work. When the smoke gets out, it stops!

Print this page

To reply to this topic, you need to log in.