Why isn’t my USB to Serial cable working?

Simply plugging in the device may not always work. You may need to download the drivers for the hardware from our website prior to installing the device.

Otherwise, please contact Support.

How can I troubleshoot my CNC that is not communicating?

There are several steps to verify if a CNC is communicating, or if it is a CNC that is not communicating. You will first want to verify the RS232 cable is connected to each device securely. There maybe two ports on the controller or appear to be a port on the front of the unit, however, this may not be connected inside the machine. CNC communication troubleshooting can be time-consuming and frustrating, so we recommend following these hints below:

You will first want to verify the RS232 cable is connected to each device securely. There maybe two ports on the controller or appear to be a port on the front of the unit, however, this may not be connected inside the machine. CNC communication troubleshooting can be time-consuming and frustrating, so we recommend following these hints below:

Hint 1:
Verify the information is coming out of the communication port by placing an RS232 mini tester directly on the back of the computer, sending a file from the CNC control, and viewing the status of the tester. This device should show RD or TD activity when the control is sending to the computer. If you do not have this tester, you can contact SFA and purchase this valuable tool, as it will save time and identify if the CNC controller or PC is failing.

Hint 2:
Make sure you are on the correct port on the back of the computer. Remove any switchboxes, adapters, misc. cables and try connecting directly to the computer serial RS232 port. If there is only one port, verify the COM port number in the device manager and match your software to this port. There maybe be various RS232 communications on your PC and you’re unaware of this port. If there are various ports, try moving the cable to the other ports.

Hint 3:
Verify the voltages on the cable are within the RS232 limits. If you are running an Ethernet cable as a serial cable, you may run into serious reliability issues. Ethernet cable (CAT5/6) cable is designed for networking computers and should not be substituted for RS232 cable to interface to a CNC control. You want to adhere to the machine tool builder’s specifications and not damage the port due to improper grounding or shielding. SFA does not sell or recommend Ethernet cable as RS232 as this normally will void warranties and damage equipment.

Note: SFA’s RS232 cable uses 8 wires, 3 levels of shielding, and has been proven to handle cable runs over 200+ feet. If you need a lengthier connection, consult our Account Executives for wireless or Ethernet solutions.

What is RS232?

The RS232-C interface was developed for a single purpose. This purpose is stated by its title:

“Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange.”

Every word in the title is significant: it describes the interface between a terminal (DTE) to a modem (DCE) for the transfer of serial data.

Short for Recommended Standard-232C, a standard interface approved by the Electronic Industries Association (EIA) for connecting serial devices. In 1987, the EIA released a new version of the standard and changed the name to EIA-232-D. And in 1991, the EIA teamed up with Telecommunications Industry Association (TIA) and issued a new version of the standard called EIA/TIA-232-E. Many people, however, still refer to the standard as RS232C or just RS232.

Almost all modems conform to the EIA-232 standard and most personal computers have an EIA-232 port for connecting a modem or other device. In addition to modems, many display screens, mice, and serial printers are designed to connect to an EIA-232 port. In EIA-232 parlance, the device that connects to the interface is called a Data Communications Equipment (DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).

The EIA-232 standard supports two types of connectors, a 25-pin D-sub type connector (DB25) and a 9-pin D-sub type connector (DB9). The type of serial communications used by PCs requires only 9 pins so either type of connector will work equally well.

Although EIA-232 is still the most common standard for serial communication, the EIA has recently defined successors to EIA-232 called RS422 and RS423. The new standards are backward compatible so that RS232 devices can connect to an RS422 port.

How is RS232 different from parallel?

In the past, most IBM PC and compatible computers were typically equipped with two serial ports and one parallel port. Although these two types of ports are used for communicating with external devices, they work in different ways.

A parallel port sends and receives data eight bits at a time over 8 separate wires. This allows data to be transferred very quickly; however, the cable required is bulkier because of the number of individual wires it must contain and cable distances are generally very short.

Parallel ports are typically used to connect a PC to a printer and are rarely used for much else. A serial port sends and receives data one bit at a time over one wire. While it takes eight times as long to transfer each byte of data this way, only a few wires are required. In fact, two-way (full duplex) communication is possible with only three separate wires – one to send, one to receive, and a common signal ground wire.

How is RS232 cabling pinned out? What does each pin do?

The number preceding each signal name corresponds to the pin number defined in the standard.


  1.     Received Line Signal Detect (Carrier Detect)
  2.     Received Data
  3.     Transmitted Data
  4.     Data Terminal Ready
  5.     Signal Ground
  6.     Data Set Ready
  7.     Request to Send
  8.     Clear To Send
  9.     Ring Indicator


  1.     Protective Ground
  2.     Transmitted Data
  3.     Received Data
  4.     Request to Send
  5.     Clear To Send
  6.     Data Set Ready
  7.     Signal Ground
  8.     Received Line Signal Detect (Carrier Detect)
  9.     +P (for testing only)
  10.     -P (for testing only)
  11.     (unassigned)
  12.     Secondary Received Line Signal Detect
  13.     Secondary Clear To Send
  14.     Secondary Transmitted Data
  15.     Transmission signal element Timing
  16.     Secondary Received Data
  17.     Receiver Signal Element Timing
  18.     (unassigned)
  19.     Secondary Request To Send
  20.     Data Terminal Ready
  21.     Signal Quality Detector
  22.     Ring Indicator
  23.     Data Signal Rate Selector
  24.     Transmitter Signal Element Timing
  25.     (unassigned)

What is causing my serial RS232 cable problems?

Here are some options, but we definitely recommend you contact our Support team.

Connecting pin one (earth ground) at both ends:
This creates a giant antenna, earth ground should only be connected at one end and generally at the device that is most grounded.

Using CAT5 cabling:

While CAT5 cable might work for a short while for RS232 communications, it is not designed for that application. CAT5 cable is twisted pair cabling for ethernet. Because it is designed for lower voltages and data that is checked packet-by-packet, CAT5 ethernet cable used for serial purposes can potentially cause a scrapped part by dropping data, short out the machine because of improper voltages and signals, and even crash a machine by sending bad data or omitting it. CAT5 cable is never recommended for RS232 communications. We recommend Predator Grizzly cable for RS232 serial communication because it is designed for this exact purpose and will prevent loss of data and interference issues.

Cable is not properly jumpered at CNC end:
Most CNCs will require jumpering of some sort. Most common is for pins 4 & 5 plus pins 6, 8 & 20.

Using unshielded cable:
Unshielded cable should never be used in a machine shop environment.

Twisted pair cabling not properly balanced:
If you use twisted pair, pins 2 and 3 can not be on the same pair. It is also recommended that you ground the other half of the pair for pins 2 and 3.

Cable with too few wires:
Most CNCs will only need TD, RD and SG, but some will require hardware handshaking and thus require more wires.

Improper positioning of cable:
Do not strap RS232 cabling to electrical conduit in the shop.

Incorrectly pinning out a DB9 to DB25 cable or adapter:
See the chart above, note that pins 2 & 3 are opposite from DB9 to DB25.

Can you make your own Grizzly Cable?

Sure, but it may not be worth the effort in research, soldering, testing, and cost of parts.

Normal serial cable is sensitive to interference and signal loss. We carefully engineered, manufactured and tested Grizzly cable to fit the needs of a shop floor environment.

Furthermore, your time is valuable and you have better things to do! Why not consult with experts?

Is a Grizzly cable just an Ethernet (CAT-5) cable?

No, a Grizzly cable is far different than commercial Ethernet cabling.

A CAT-5 cable is typically un-shielded, provides a balanced signal, and uses different wiring intended for a computer, as opposed to a CNC control.

The Grizzly Cable is shielded, with an ultra-low capacitance, uses unbalanced signals, and is wired for RS232 signals to be sent to and from CNC machines.

CAT-5 cables are unreliable, able to pick up RF noise very easily and have limitations regarding baud rate when running beyond RS232 specifications.

How are Grizzly Cables shielded? A Grizzly Cable has four layers of shielding.

First, each wire is individually shielded with a polypropylene jacket. Second, an aluminum foil wraps around all of the pairs. Third, a metal braid shield is wrapped around the foil. Finally, a thick but flexible PVC jacket is wrapped around the metal braid.

The quality and strength of the shielding included with Predator Grizzly cables protect your data from RF noise and electrical interference. We highly recommend that you do not attempt to make your own Grizzly Cable.

How do I use the RS232 mini tester from SFA?

Shop Floor Automations recommends a DB25 RS232 mini tester with every hardware order. We terminate our RS232 cabling with RJ45 connectors and have different adapters to tailor to each installation. Depending on your machine, you may require different pinouts on the adapters.

To test the correct pin-out in the cabling:

  • Connect the cable to the PC, Hub, or switchbox.
  • At the other end of the cabling, where the control would normally plug in, connect the tester. Without the tester plugged in the machine, you should see have “RD” lit and “TD” unlit.
  • For most applications, the other lights do not matter. If you plug the tester into the CNC machine control, you should then see “TD” lit.

If you have any variation of this, your machine will not communicate properly.