With thanks to a commenter on a previous article, I’ve had a close look at the ribbon cable which connects the TC35 module to the UART board. After a bit of fiddling with the (tiny) clamps which hold the cables in I was able to release the cable. At least one of the contacts appeared damaged, so I trimmed the end off each cable with a sharp pair of scissors and reconnected it. This picture shows the two boards disconnected with the ribbon cable in the middle. Normally the two boards are on top of each other and I imagine that the ribbon is under a bit of strain.
Initially there was no change – but then I tried reversing the TX & RX pins (ie connected TXD – TXD rather than TXD – RXD) and all of a sudden- success! I can now control the module through AT commands using my new USB device.
So thinking I had solved the problem, I rewired the cable to the alarm panel (swapping TXD & RXD) and tried it – but once again with no success. I’m not quite sure what I’m doing wrong, or I suppose it might be that there is some other reason (maybe incompatible voltages) why this unit doesn’t work. I did wonder if it was because my SIM has no credit but it should still report signal strength and it does so if I try sending AT+CSQ directly.
So if anyone else out there is struggling with one of these modules – try trimming the ribbon cable and swapping over the TXD & RXD pins.
Meanwhile I have received the new board from China – which is tiny by comparison with the one I have. I know from Andy that this definitely does work, so I’m now waiting for the regulator board to arrive and I’ll give it a go.