Retrieving data from Agilent 34401A Digital Multimeter over the RS232 interface

Question

I want to interface Agilent 34401A Digital Multimeter over the RS232 interface. The settings (baud rate, parity, number of start and stop bits) are set to be same on multimeter, pc (device manager) and on the application that I use for terminal. Programming language is SCPI.

I successfully control the multimeter, that is I execute the commands such as: syst:rem; :meas:volt:dc? 10, 0.003; :conf:volt:dc 10, 0.003; :sens:volt:dc:rang 100; but I get no response over RS232 interface from my multimeter.

For example when I execute the command :meas:volt:dc? 10, 0.003; the measurement is performed and the voltage is displayed on multimeter’s display, but the value is not sent over RS232 to my terminal. No response what so ever.

In the manual for the multimeter there is a statement after each command -- “bus enter statement". Relevant extract provided below.

I could not find any reference or explanation what is meant by this. Does a special command exist for retrieving data from the multimeter?

:read? makes no effect on the multimeter.

Useful hint for someone who is also trying to set up RS232 communication with this device: Even if in the manual the colon preceding the SCIP command is not given, the multimeter returns “unknows header” error if it is not present.

Answer 1

The problem was in the way I send a command over terminal. After a command the multimeter expects line feed character, in PuTTY this is <ctrl-j>.

The code below successfully gets response with a value of a voltage.

syst:rem; 
:meas:volt:dc? 10, 0.003;
<ctrl-j>

I found solution reading through this forum question: https://www.eevblog.com/forum/testgear/34401a-serial-interface-problems/

Answer 2

I had to deal with this matter today. Briefly, the matter is that Agilent has uses an unconventional hardware flow control handshake.
Long story short, the instrument needs to see a high voltage on its DSR input.

If you're using C#, keep note it is possible to set DTR (which connects to DSR using a regular null modem cable) to either 'high' or 'low' regardless of using or not Hardware flow control.

Thus, to make it work:

• In Visual Studio, set your COM port to the instrument's baud rate and parity.
• Set your flow control to NONE.
• However, set and leave the DTR line to "high" (my line shows about 5.8 volts). You may trigger it low and high to confirm you can control it. This is recommended since this behavior may vary according to your specific serial port hardware or USB to RS-232 adapter.
• Remember, prior to issuing commands set the mode to 'remote' using the command SYSTEM:REMOTE.
• Also remember to finish your commands with \r\n.

Here's some functional code:

 
/* 
Agilent A34401 Test Program
---------------------------
Copyright (C) 2021, Diego Velez
The code below is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License 
*/ 

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO.Ports;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AgilentTest
{
    class Program
    {
        public static SerialPort sp;
        public static Stopwatch sw = new Stopwatch();

        static void InitSerial()
        {
            sp = new SerialPort("COM16", 9600, Parity.None, 8, StopBits.One);
            sp.Handshake = Handshake.None;
            sp.DataReceived += new SerialDataReceivedEventHandler(sp_DataReceived);
            sp.ReadTimeout = 5000;
            sp.WriteTimeout = 5000;
            sp.Open();
            sp.BreakState=false;
            sp.DtrEnable=true;      // Important: this tells the instrument we're ready to receive data,
                                    // in spite of not actually using hardware flow control
        }

        static void sp_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            SerialPort mysp = (SerialPort)sender;
            string indata = mysp.ReadExisting();
            Console.Write(indata);
        }

        static void Main(string[] args)
        {
            InitSerial();
            sw.Start();
            int seconds;
            bool flag_running=true;
            int time_stamp_last_command=-1;
            sp.Write("SYSTEM:REMOTE\r\n");
            // To leave the loop, find a way to set flag_running to false
            while (flag_running)
            {
                seconds=(int)sw.ElapsedMilliseconds/1000;
                if ( ((seconds % 10) == 0) && (seconds!=time_stamp_last_command) )
                {
                    time_stamp_last_command=seconds;
                    Console.WriteLine("{0} sent command", seconds);

                    sp.Write("MEASURE?\r\n");
                }
            }
        }
    }
}

Answer 3

I found that to get the MEAS:VOLT:DC? command to work with my 34401A, I first had to put the system into remote mode.

So, in LabVIEW I first sent

SYST:REM\n

then I sent

MEAS:VOLT:DC? 10,0.003\n

over my VISA port. When I read back the buffer I got the voltage as desired as a text string.

Note that the \n is the LabVIEW "Linefeed" string constant, I concatenated that to the end of the main part of the string using the concatenate string block.

This was using a RS-232 to USB converter from plugable.com, with no cross-over (null-terminal) or anything else.

Answer 4

The Agilent devices require a somewhat esoteric form of flow control - DSR/DTR. You'll need to set that up before you can read data.

Answer 5

The serial cable pin-out From:

34398A/99A RS-232 Cable Kit/Adapter Kit User's Guide

34398-90001.pdf

I remember wiring some of these up when I was in the same situation...

Answer 6

In addition to insert a Line Feed after each command, I had to also set DTR (Data Terminal Ready) to be able to recieve any response.

Answer 7

Take a look at the linked document. See if the fetch? command returns anything similar to what the document shows. Also, check if your cable is wired the same as in the image on the last page. Using HyperTerminal with Agilent General Purpose Instruments