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Section 7: How to Identify Pinouts, with or without a Blinker Box

Previous Page: Section 6: Intermittent Polling on 202 and FSK Lines
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If you have problems connecting devices together, it's always a good idea to double-check your pinouts. But sometimes the pinouts are unknown - either the pinout documentation is missing, or the port doesn't actually behave the way the documentation says it's supposed to.

In those cases, you can identify the pinout with just a little bit of detective work.

Section 7a: Identify Pinouts with a Blinker Box

Blinker boxes are convenient for detecting pinouts.
A blinker box is a great convenience for detecing pinouts.

Testing pinouts is easier if you have a blinker box, a small electronic device that connects to a port and lights an LED for each pin that transmits a signal.

  1. Most blinker boxes only light LEDs for pins that transmit data. For RS-232 ports, these are TXD, DTR and RTS. However, some blinker boxes will light green for transmit pins and red for idle and receive pins.
  2. Plug your blinker box into the near-end port of the problem connection and write down the status of the LEDs.
  3. Then plug the blinker box into the far-end port and double-check the LED status there.

Note: even with a blinker box, it will still take some detective work to identify for certain the various transmit pins - for example, if you have two transmit pins, which one's DTR and which one's RTS? You can use trial and error to match each transmit pin with its corresponding receive pin on the other side, such as RTS to CTS.

  1. You may find that the near-end and far-end pinouts don't agree. But now that you've identified the real pinouts, you have a guide for rebuilding your cables.

Blinker boxes are convenient, but they're not absolutely necessary. You can also identify pinouts with more common tools.

Next Page: Section 7b-c: How to Identify Pinouts with a Butt Set or PC
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