Monday, July 22, 2013

RS232 to Control LXI Instruments

Instruments that were designed more than a decade ago included an RS-232 serial port as an I/O interface. Using RS-232,  users were able to automate their instruments' measurements and configuration settings. However, RS-232 is no longer being offered as an interface on newer instruments. Some users have been reluctant to upgrade their systems with newer instruments because they won't be compatible with their system's RS-232 serial I/O. Users still want to be able to communicate to their instruments using RS-232. With these users in mind, I believe that I have found an off-the-shelf solution for RS-232 controllers to communicate with modern test instrumentation.

I wanted to find a converter that would enable communications between my RS-232 controller to a 34461A Truevolt DMM which has USB, LAN, and GPIB (optional) I/O ports. I also wanted the configuration process to be simple and painless. I didn't want to spend days trying to figure out how to get the converter to work.

The 34461A is targeted as a replacement for the 34401A. The 34401A has RS-232 while the 34461A does not. This post should help some of you that are looking at migrating to the 34461A but the lack of RS-232 support was a barrier. The techniques detailed in this post should be applicable to most Agilent LXI instruments.

Perhaps you have looked at RS-232 to USB converters as a solution but those typically connect the USB side to the PC controller and RS-232 to the device. This doesn't work in this case since you want the RS-232 on the controller side and USB on the instrument side. USB relies on a host/device pair which requires that the host have drivers to communicate with the device. Most manufacturers recognize the difficulty in having a USB converter act as the host since the converter will need to have upgradeable drivers for any USB device that the user wants to install. I haven't found any products that will allow users have RS-232 on the host side and allow custom drivers to talk with the device.

 RS-232 serial controller to communicate to a LAN instrument
Many Agilent instruments offer LXI certified Ethernet ports in addition to USB or GPIB.  Since TCP/IP Ethernet is essentially a handshake protocol without a defined host or device, this matches well with the RS-232 serial protocol for our I/O purpose. After searching on the internet for a little bit I found a few promising RS-232 to TCP/IP-Ethernet converters that might work for  RS-232 communication to LXI instruments.

I purchased the Startech NETRS232_1 from since it had some good reviews regarding reliability and driver support. You might want to purchase something that is rack mountable if you need to permanently mount your converter. Startech has a few other converters that offer the same functionality in a different form. I haven't purchased the Perle or Lantronix products although I believe that they will offer the same functionality.

Configuring the Converter and DMM

The default configuration for these types of converters enables the user to connect to a single serial device from multiple Ethernet devices. They are targeted at networking serial devices such as a serial printer. This is not our use case. We need to be able to connect a serial device (our controller) to a single instrument. In order to configure the NETRS232_1, I had to connect the converter's lan port to my PC using a crossover Ethernet cable. I then ran the IP Extender Manager software that came with the device. Once the software found the converter,  I had to activate and unlock the device (accomplished by right clicking on my device in the software). This allowed me to access the properties of the converter. 

I kept the converter's default network settings of:
Static IP:
Subnet mask:
Gateway address:
Name: [empty]
Workgroup: [empty]
DHCP detection: Disabled

Configuring RawClient Mode
The NETRS232_1 has a RawClient Port Mode that will allow you specify a target IP address and TCP port for the converter to use. To change to that mode I right clicked on my converter in the right hand pane and chose properties. Then in the Target Settings tab, I entered an IP address of for my DMM (more on that below) and specified TCP port 5025. Port 5025 is the port that should be used for LAN socket connections on most Agilent LXI instruments and is important for this application.

The converters RS-232 serial settings I left at the default.
Bits per second: 9600
Data Bits: 8
Parity: None
Stop bits: 1
Flow control: None
All other settings were left at the converter's default RawClient settings.

On the DMM side I needed to configure the LAN settings to match my converter's setting. Getting into the Utility>I/O config>LAN settings>Modify Settings screen allowed me to change to Manual (static IP) mode. I then changed the Subnet Mask to match the converter's default The IP Address was a little bit more tricky and I was only able to get the converter to communicate to my DMM if the first three fields matched the converter's static IP address i.e. put it in the same subnet. This shouldn't be a problem since the only IP devices will be the converter and the DMM. Since the converter's IP address was, I chose my DMM's address to be Note that this address has to match the address that we set in the converter's RawClient's Target IP Address step above.

Testing the solution

Once my DMM and my converter were configured, I tested it out using the RS-232 port from my PC. Remember, from the PC's standpoint it only knows about RS-232 and does not know that the 34461A is a LXI instrument. The easiest way for me to communicate via RS-232 was to use Agilent Connection Expert (ACE) from the Agilent IO Libraries Suite. I opened up ACE and configured my COM1 port to the same RS-232 serial settings as the converter's. The picture below shows that ACE is able to communicate to my 34461A via RS-232. ASRL1::INSTR is the convention that Agilent uses for addressing RS232 communication on the COM1 port.

I also tried to do some basic configuration and readings. Using Agilent Interactive IO, I was able to do single readings and 10 readings per sample. This proved to work well and I didn't have any issues with this. I won't be able to get maximum DMM reading and transfer rates with this converter due to the limiting transfer speeds of RS-232.

Additional Tips For This Solution

I did find a few small things to be aware of when working with this adapter.

  • As mentioned before, I needed to set the IP address of the DMM to be in the same IP address  range of the converter. Even though I had set the same subnet mask, I could not get the converter to communicate to the DMM using the DMM's default Auto-IP address of I needed to set the IP address to be within the same range as the default settings of the converter at, to put it onto the same subnet. This meant that I needed to set the address to,where xxx is any number between 0 and 255.
  • I also found that the converter took a little bit of time to apply it's settings. By this, I mean that after I would change the settings I could not communicate to the DMM right away. This led me to believe that I had a setting wrong. An example of this is when the converter didn't change the target IP address right away. After about 2-3 minutes the converter would apply its settings and I could communicate.
  • This solution shouldn't need IT approval for using the DMM's Ethernet port. Since this is a direct connection from the PC to converter to the DMM, all Ethernet traffic is isolated to the converter and DMM.
  • I didn't try to set the converter to DHCP mode in order to put it onto the LAN. Most applications that need RS-232 are looking at direct connections from the controller to instrument. However, I believe that DHCP mode would work provided that the IP address of the DMM/instrument is static so that you can configure the converter's Target IP address.Use of DHCP and you LAN might need your IT department's approval.

As a proof of concept, it looks like the RS-232 to Ethernet converters will work if you that need to use RS-232 with modern instruments. The configuration process took me about 1.5 hours to figure out and understand the subtleties. With the steps that I documented here, it should take less than 30 mins to setup in the future. Not too bad for an off-the-shelf solution.

Tuesday, July 16, 2013

Truevolt DMM Teardown, Review, and Additional Truevolt Topics

Today I'd like to highlight some videos that were published by the EEVblog. For those of you that don't know about Dave Jones and his blog, has become one of the premier locations on the web for reviews and tear downs of Test and Measurement equipment.

The EEVblog forum is very active and many of the contributors are well informed and educated on different technologies. I have enjoyed reading the forum comments pertaining to the Truevolt DMMs.

Dave Jones has done a really great job and his review is overall very positive for Agilent's new DMMs. He's put a lot of work into both videos and provides a lot of insightful information for an independent review.

EEVblog #489 - Agilent 34461A Multimeter Review

EEVblog #485 - Agilent TrueVolt 34461A Multimeter Teardown

These videos and forums have brought up some topics that I think that I can address for the Truevolt DMMs.

How to get 1000 readings per second:

In order to get the fastest reading speeds you need to set the fastest integration time. For the 34461A, this is 0.02 NPLC. Also you need to turn off auto-zero and auto-ranging (by setting an explicit range). In the Acquire menu, you need to set the trigger delay to <90 micro seconds, a delay of 0 will get you better than 1000 readings per second.

With these settings you can get the maximum reading speed. The digit masking feature does not increase the reading rate, it only hides digits of resolution to provide easier reading for you. If there is enough interest I can provide a video to test this in the future. Let me know through a comment below or email me directly at

Recalibration required after removing shield:

The printed circuit board actually has a label next to the shield that states "Recalibration required after removing shield".

There is some doubt from users that a recalibration is actually required. I spoke with the R&D engineers about this and their statement is below:
"Removing the shields affects AC flatness calibration. The adjustment calibration procedure compensates for small changes in capacitance from high impedance nodes to the shield. By removing and replacing the shields, these small capacitance values are changed and could affect the DMM's flatness calibration values." 

How can the 34461A be a replacement for the 34401A without RS-232?

Its true that the 34461A does not have an RS-232 port for I/O communication and this will deter many 34401A users from upgrading. I'm working on another blog post to address RS-232 users and I think that it will meet the needs for those of you that need RS-232 to control your DMM.