USB-C Serial Cable

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USB-C Serial Cable

Connects a USB-C iPad to RS-232 serial devices at baud rates up to 115.2 Kbps.

This cable is used in conjunction with the Redpark Serial SDK to create iOS apps that communicate with a serial device. It is licensed under the Made for iPod (MFI) program for private deployments under the terms of an enterprise, education and personal iOS developer license.

Under current Apple policy this cable may not be used with apps sold on the App Store.  However apps may be deployed via Apple Business Manager and Apple School Manager.

  • C4-DB9V
  • $69
  • 013964221671
  • One year
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Serial Interface

  • Male DB9 connector
  • Max Baud Rate:  115 Kbps


  • For use with all iOS devices with USB-C connector
  • Requires iOS 14 or later


  • Length  6 ft


If your Redpark cable is not working properly try these troubleshooting steps below. 

Power your iPhone OFF and then ON again. (Do not simply put the iPhone to sleep. Power it completely OFF and then ON.)

If the problem continues to exist…

Confirm that your iPhone/iPad detects the cable:

  • Connect your cable to the iPhone
  • Open the Settings app on your iPhone
  • Select “General” then select “About”
  • Scroll down the list.   “Serial Cable” will appear near the bottom of the list if the cable is detected by iOS).

If the iPhone IS detecting the cable:

Check whatever app you are using to make sure that the communication settings (baud rate, parity, etc.)  are properly configured. Check with the app developer if you have questions about the proper configuration.

If the iPhone is detecting the cable and the communication settings are correct but you are still experiencing a problem — perhaps the cable has developed a short in its wiring.   If the cable is within the one year warranty please return the cable to Redpark to be tested.  

If the iPhone IS NOT detecting the cable:

  • Make sure the case on your iPhone is not preventing the cable from being fully inserted into the USB-C connector of your iPhone.
  • Use a toothpick or safety pin to remove any debris that might have accumulated in the USB-C connector of your iPhone.
  • Try the cable on a 2nd iPhone or iPad to see if you get the same results. If the cable works OK on the 2nd iOS device then it is likely that cable is good.  If the cable does not work on the 2nd iOS device and the cable is within the one year warranty please return the cable to Redpark to be tested. 

 If you choose to email Redpark for assistance, please email “”.

What kinds of devices have serial ports these days?

Ham radios, dive computers, point of sale devices, Arduino and other micro-controller platforms, scientific instrumentation, data loggers, RFID readers, automobile engines, routers and other networking devices, glucose meters, fire alarm panels, irrigation controllers, weather stations, specialty printers, scales, temperature sensors, flight loggers, satellite radios, model cars (models boats, planes and rockets too), CNC machines and more…

What version of iOS is required?

The C4-DB9V requires iOS 14 or later.

Is there a way to connect to devices using TTL (logic level)?

Yes, by connecting an external RS-232 to TTL adapter to the DB-9 connector on our cable.

Can I power the iOS device using this cable?

No. Please use our Serial+Power Adapter (L4-SERP) instead.

Can I power the serial device using this cable?

Power is not supplied through an RS232 connection. Some very low power devices make use of the RS232 signal lines to supply power. This usage of the RS232 connection is outside of the RS232 specification and may not work in every instance. It is left up to the user to determine if the RS232 output signals will supply enough power for a particular application. Currently, the C4-DB9V uses a MAX3243 or similar driver to convert logic level signals to RS232 level signals. The specifications for the MAX3243 may be used as a rough guide to determine if the signal levels are appropriate for your application. Redpark reserves the right to change the RS232 driver circuitry at any time.

What apps does this cable work with?

The cable and our Serial SDK are provided to you so that you may write your own apps.

Does the Adapter work with apps available at the App Store?

Apple allows apps to be deployed via Apple Business Manager and Apple School Manager for use with the USB-C Serial Cable. However apps may not be deployed via the public Apple App Store at this time.

How can I tell if my iOS device recognizes the cable?

Open the “Settings” app on your iOS device. Select “General”, then “About”. If the cable is detected by the iPhone, “USB-C Serial Cable” will appear in the list shown.

How do I know what version of firmware the cable is using?

Once you have located the USB-C Serial Cable in the “Settings” App, select the USB-C Serial Cable to view its parameters.