TTL Connector for Linksys AC Series Routers
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The BEST TTL to Serial Adapter that can be purchased for use with a Linksys AC Series Router is based on an FTDI chipset. Of all the adapters available, the best one is from a company named DSD Tech, model SH-U09C.

TTL to Serial Adapter Model SH-U09C from DSD Tech
TTL to Serial Adapter Model SH-U09C from DSD Tech


This article is about finding the best TTL to Serial Adapter available on the market today. It is written in regards to the Linksys AC Series of routers. But the information can applied to Arduino, Raspberry Pi, and other similar single board computers or routers.

The Objective

Finding, Purchasing, and Connecting a TTL to Serial Adapter to a Linksys AC Series Router for system console access.

A Brief Orientation

A summary of acronyms, brand names, models, etc.

The Issue

Finding the right TTL to Serial Adapter for the job

The Solution

The model SH-U09C from DSD Tech is the best TTL to Serial Adapter.

Product(s) Review

The rational for deciding the best TTL to Serial Adapter, the SH-U09C is explained.

The Journey

Many, many TTL to Serial Adapters were tested, with the SH-U09C from DSD Tech coming out on top.

The Pollution

There are so many, many articles on this subject that have no substantive information and are not helpful.

More Information

There are other models available too.

A Brief Orientation

TTL is an acronym for Transistor-Transistor Logic (not Time To Live which is related to DNS). The definition of TTL by Wikipedia doesn't do a great job of explaining how TTL relates to serial communication method that takes place over copper wires. And technically, when referring to a Serial Adapter, the more accurate terms are RS-232 or V.24.

The Linksys AC Series Routers, sometimes referred to as the new WRT Series of routers, consists of several models. They are the WRT1900AC, WRT1900ACS, WRT3200ACM, and WRT32X. Some of the models have additional model designations such as version 1 (v1), version 2 (v2), and references to various other products such as the Microsoft X-Box.

The Issue

The Solution

DSD Tech

The official DSD Tech website does not do justice to the care they put into their products. Their Amazon Store site is much, much better than their official website. Maybe they're concentrating on making good products instead of a good home page.

More Information

TTL and Serial / COM (AKA RS-232)

A good explanation of Serial Communication along with a comparison between TTL and RS-232 is here: RS232 vs TTL: RS232 vs TTL: Beginner Guide to Serial Communication (the RS-232 VS TTL is at the end of the article) *

Other Models

Another model to keep in mind is the newer SH-U09C2 from DSD Tech. It adds support for 1.8V signalling.

Another model (unknown number) in a different form factor is from a vendor named DIYMall. This style might be more useful for Raspberry Pi and Arduino applications. It will still fit inside of a Linksys AC Series Router, but the DSD Tech products fit better.

The Real Pin # 1

The Issue

The diagram(s) referenced and reused by many, many people for the TTL connector on the Linksys AC Series routers is incorrectly labeled. The numbering of the pins is reversed. Specifically, the numbering showing the pins labeled as 1 through 6, left to right, as looking at the board from the top / front. It should be numbered 6 through 1. Although, the perspective of the diagram could be looking from the top / rear of the board, which would make the numbering correct.


Many website tutorials, YouTube videos, diagrams, and even images on the OpenWRT website show the TTL Serial Port for the AC series of routers numbered from left to right, starting with "pin 1", as viewed from the top / front of the circuit board. This is NOT correct, although both the names / labels of the pins and what their function is (Ground, TX (Transmit), and RX (Receive)), are correct. It is an industry standard to identify "Pin 1" on a circuit board in several manners. The most common methods includes a triangle printed on the circuit board closest to "Pin 1" and a square solder connection (as viewed from the bottom). Another convention that is typically followed, but is not an absolute rule, is putting "Pin 1" closest to the nearest edge of a circuit board. The AC Series of routers all have a square solder connection on the bottom and a triangle printed on top of the circuit board for "Pin 1" on the right side of the connector, as viewed from the top. Since there is no pin out standard for that type of connector, the manufacturer (Linksys in this case) gets to define where "Pin 1" is located. And per the triangle marking, pin closest to edge, and square solder connection pin number one is located as indicated by the below image. PERIOD.


This is not intended to be any sort of attack on the nice people who have done all of the difficult work to figure things out, document it, etc. Somewhere along the way, someone made an innocent mistake, and it has been propagated quite a bit. This short diatribe on the Pin # 1 subject is intended to point out the issue in the hope that it may one day be corrected. It's also an interesting lesson in how once incorrect information gets out there, it's difficult to correct.

Side Notes

* As good as the article is, the title really buries the lede: RS232 vs TTL: Beginner Guide to Serial Communication(shouldn't RS232 VS TTL be the first paragraph?). And having a desire to link a reader to the specific text and anchor tag / id was searched for, but didn't exist. Hence the need to reference the pertinent section at the bottom of the article. Without an ID or NAME attribute, there's no way to link to that point on a page. And it's coming from a Word Press site. That brings up the idea of automatic ID tags for posts and pages (and paragraphs as it turns out)

And the Serial Header Pin Out chart / diagram in the OpenWRT documentation on the OpenWRT site is incorrect.

The Pollution

Try searching for something like best, ttl, serial, adapter. All of the top results list a lot of TTL to Serial Adapters. But none of them state why. And it's obvious a lot of them are written in an automated fashion, for the purposes of scoring high on a search engine's results. Not many of them care. For instance, this example: It's just a long list of products. No testing results, no comparisons, no methodology, just a bunch of aggregated information. Now compliments do need to be given to the author, Mike Davis, in term of SEO refinement and being listed as a top search result. But for substantive content, that article is like a candy bar. It might taste good, but in terms of nutrition (IE, equating nutrition to knowledge), it's useless. Case in point, a quote: "Thereof, can you convert RS232 to USB?" What? The answer to that question is even more robotic (and useless).