GM862 and the damn V.24 naming of TX/RX

Wireless & IoT
1

Hello,

The first time I (tried to) use a GM862 modem, it pretty much caught fire. After replacing it, I sorta missed the fineprint that says “TX/RX are named according to V.24”, and it didn’t work out either (lol). Basically, V.24 says that the TX and RX lines aren’t named TX and RX, instead “TX” on the modem side means “Connect to TX on the DTE”. So to someone that’s played with serial before, TX>>RX and RX>>TX doesn’t work! (pulled my hair out over this for quite a while heh)

Anyway that was a couple of years ago now, i’ve since learned to read the damn manual :wink: - the problem i’m facing now, is that I need to implement hardware flow-contol. Now i’ll be damned if I can figure out how to do this on GM862’s - Generally its just a matter of wiring up RTS on the modem to CTS on the DTE side, and CTS on the modem to RTS on the DTE side (plus CD / RI / etc if you’re feeling adventurous, or you have enough IO pins on your micro heh)

But then there’s that damn V.24 thing again. The manual doesn’t mention changes to pins other than TX/RX, so I guess we can ASSUME that RTS/CTS are unchanged, and we can cross them over on the DTE side.

But then AGAIN the manual says RTS goes to rts_uart, CTS goes to cts_uart (it says that VERY vaguely though), so that’s straight through and not crossed over (ie: V.24)

And so, now I ask you… What the hell? (Page 31 of the GM862-GPS user guide, 1vv0300728 Rev. 2 - 15/09/06 for those playing at home)

If you’re curious, this is what the current board looks like: http://www.horizons-edge.com.au/DSCF3430.jpg - I lost my PCB layout AND firmware during a format (what are the odds of BOTH backups failing!) so I need to start over. The new one will be similar, but with four EEProms and additional level shifting circuitry - it’s currently 70mm x 50mm

I’d rather not do a trial/error thing with RTS/CTS, it’s quite a pain to homebrew another board like that :\ I’m running out of supplies, and my printer is having trouble with its fuser… So I can probably make just a couple more before it dies on me

Any help would be greatly appreciated

-BaudThief

2

It can be confusing. The way that I read the GM862-GPS Hardware User Guide r6, it shows that the C108/DTR and C106/RTS are inputs and should be connected to the DTR and RTS pins of the DTE. The C106/CTS, C107/DSR and C109/DCD are outputs and should be connected to the CTS, DSR and DCD lines of the DTE. These, of course, need to be routed through a level-shifter if you are hooking it up to a PC.

Hope this helps,

Greg

3

Hey!

Thanks for the response, I thought the exact same and routed it that way anyway heh (looks as though you’re right!) - only problem I’m having now is the Ring Indicate rail staying high eternally (openned a new thread for it here: viewtopic.php?t=7092)

It’s a bit strange to say the least! disconnected everything and tried a different modem, same deal - any clues?

Thanks again!

-BaudThief