Connecting a cheap digital multimeter to your computer using Serial Extender

Keywords: 	 digital multimeter serial port

In various electronic stores, including Tandy RadioShack and many others you can purchase digial multimeters and other equipment with a serial interface for connecting to a PC.

Using the WinBatch Serial Extender you can connect this equipment to your PC and read data from the device.

However in attempting to do this we have found that it is not as obvious as it looks.

It turns out that most of these devices "cheat" on some of the electronics required, and that the serial port has to be configured in a non-standard manner for the device to function.

In particular...the serial interface uses voltages between -10.0 and +10.0 to signal the ones and zero of the serial bit stream. Instead of adding the expensive electronics to the device to generate -10.0 to +10.0 volts off of the (usually) 9 volt internal battery, the software must set the card up to deliver these voltages to the device on otherwise unused control lines, so that the device is really stealing the voltage from the control lines from the PC's serial card.

These devices seem also to be slow, in that they sample the voltage every second or so, instead of instantaneously, so that when a reading is requested sometimes it takes a second or two for the device to deliver the reading on the serial port.

The following script can read data from the RadioShack 22-805 as well as the Metrex ME-11 24-Range LCD Digital Multimeter. These devices run at 600 baud. Other similiar devices seem to run at 1200 baud.

; Load the WIL Serial Extender
AddExtender("wwser34I.dll")

; The RadioShack 22-805 device wants a D sent to it to causes it
; to return the display reading.  That is the only command it knows.

commandstring = "D"


; Open the COM port  600 baud, 7 data bits, no parity, one stop bit
; no handshaking 
port=pComOpen("COM1",0,600,"7N1","NONNON")  ; Open com port

;Set timeout to 1.5 seconds to give device a chance to respond before
;calling an error
pTimeOut(port,1500)

; Force RTS line low. to give device a source of -10.0VDC
; THis is *very* non-standrd for serial communications
pComControl(port,5,0,0,0)

; Write command string to the device
didit = pPutString(port,commandstring)

;Wait for then read response.  14 characters 
resp=pGetString(port,14)

;Close the port
pComClose(port,0)

; Display results
Message("Response is",resp) ; Display it

Exit

Devices that seem to have similiar setups are...

 
 1200 baud devices
 Metex 4650CR
 Micronta 22-182

 600 baud devices
 Metex ME-11 
 RadioShack 22-805

Article ID:   W14553

Filename:   Digital Multimeters with Serial Ports.txt

File Created: 2001:03:01:15:25:46

Last Updated: 2001:03:01:15:25:46

Database Search

TechHome

Connecting a cheap digital multimeter to your computer using Serial Extender