OpenWrt Forum Archive

Topic: TP-Link WR741ND / WR941ND / WR1043ND serial connection

The content of this topic has been archived on 7 May 2018. There are no obvious gaps in this topic, but there may still be some posts missing at the end.

Post #1
0x00
22 Jun 2010, 11:08

Hello!

I would like to build custom images of OpenWRT for TP-Link WR741ND / WR941ND / WR1043ND routers, while at the same time change kernel sources. This will surely result in some unbootable images. I would prefer a software solution by creating a u-boot saveenv partition – as I described in my post on this board a few days ago , but I can’t do this myself:

mtd0 is boot, mtd1 is kernel, mtd2 is rootfs. They don't contain a valid "saveenv", do they? But it shouldn't be too hard to create a mtd1 0x4000 big partition (squeezed between u-boot and kernel), containing a valid saveenv environment. Or rather mtd2 partition between kernel and rootfs as 0x00020000 is hard coded as kernel's start address. But that I think, this can be changed in saveenv to eg. 0x00024000, so that mtd2 kernel could start from there. Anyhow, it would be better to modify u-boot code, as 0x4000 bytes would be saved.
Link: https://forum.openwrt.org/viewtopic.php … 57#p111557

So the next best thing is serial connection. From *WRT forums I found the following specs:
WR741ND v1 - VCC (3.3V) , GND, RX, TX,
WR941ND Pin, 3.3v serial, 2  RX, 3  TX, 4  VCC (+3.3v), 5  Gnd,
WR1043ND – same levels.

My question is about 3.3V VCC, since I cannot find a converter cable that would support it. There is an “oldwiki” page on OpenWRT – http://wiki.openwrt.org/oldwiki/openwrt … al_console citing examples of TTL 3.3V prebuilt USB cables. The problem is that the mentioned FTDI TTL-232R-3V3, FTDI TTL-232R-3V3, FTDI TTL-232R-3V3-AJ and FTDI TTL-232R-3V3-WE cables have a 5V VCC, not a 3.3V VCC. There’s a mistake on the cited http://apple.clickandbuild.com/cnb/shop … egoryID=47 page, since the FTDI datasheet clearly states that:

The TTL-232R-5V-WE and TTL-232R-3V3-WE cables are both un-terminated; they are bare and tinned wires. The difference between the two cables is that the TTL-232R-5V-WE operates at +5V levels (signals and power supply) and the TTL-232R-3V3-WE operates at +3.3V levels (signals only, VCC=+5V).
Source: http://www.ftdichip.com/Documents/DataS … S_V201.pdf , pdf page 14.

I would be really glad, if someone can point me to a USB <-> TTL converter cable with open ended wires, similar to TTL-232R-3V3-WE, but with 3.3V VCC levels. Preferably, it should be available in Germany.

Thanks, and happy OpenWRTing.

Post #3
0x00
22 Jun 2010, 12:38

Thanks. I'll get the Nokia CA-42 clone cable then. The instructions in the links for CA-42 seem to be quite straightforward, and it seems that +3.3V VCC is present.

Post #4
jescombe
22 Jun 2010, 16:52
0x00 wrote:

Hello!I would be really glad, if someone can point me to a USB <-> TTL converter cable with open ended wires, similar to TTL-232R-3V3-WE, but with 3.3V VCC levels. Preferably, it should be available in Germany.

Remember you won't want to connect the 3.3v Vcc from the header to the USB-TTL cable anyway. Just ground and the signal wires...

That FTDI cable supports 3.3v logic levels, the 5v Vcc is just there to power your embedded board (from the USB bus) if you require it.

Post #5
Sidra
10 Aug 2011, 07:29

Is pullup resistior required for Tx in TP-link 741ND?? for connecting  to serial port??

(Last edited by Sidra on 10 Aug 2011, 07:32)

Post #6
hugo1789
10 Aug 2011, 09:27
jescombe wrote:
0x00 wrote:

Hello!I would be really glad, if someone can point me to a USB <-> TTL converter cable with open ended wires, similar to TTL-232R-3V3-WE, but with 3.3V VCC levels. Preferably, it should be available in Germany.

Remember you won't want to connect the 3.3v Vcc from the header to the USB-TTL cable anyway. Just ground and the signal wires...

That FTDI cable supports 3.3v logic levels, the 5v Vcc is just there to power your embedded board (from the USB bus) if you require it.

If your cable works with 5v logic levels, it outputs 5v on tx which I don't know whether that destroys your router or not. Just place a small resistor (lets say 50 Ohms or so) between the cables TX and the routers RX pin. I also cannot recommend connecting VCC, as jescombe said, it's just not nescessary. If you really insist on connecting VCC from a 5V cable, insert 2 or 3 diodes between cable and router (that should drop VCC to 3.6 or 2.9v).

If your cable is a TTL-232R-3V3-WE, that's already a 3.3V cable. Just connect GND, RX and TX and you are ready to go wink

The discussion might have continued from here.