Help with Metex ME-11 DMM RS-232 interface

[Bill McLaughlin]

Hello all. I hope this is the correct forum for this question. I did a
search on Dejanews and found some similar notes in the past.

I have a Metex ME-11 DMM with an RS-232 port and I am having trouble
getting my computers to talk to it. From looking at the manual, it
should be really simple. The manual states 600baud, 7 bits, 2 stop
bits. You are supposed to send it a capital "D" to get it to send back a
value. I tried this but I didn't get anything so I connected one of
those mini RS-232 testers with the two color leds. When I plug the cable
into the computer (an Alpha, not a PC) the TD, RTS and DTR LED's glow
green (which is positive, I think). When I then plug in the DMM, the RD
shows a very faint green as well. When I send the "D", I see the TD
blink quickly and then the RD blinks for a second or so. The meter is
supposed to send 14 ASCII characters each time it is queried.

I had tried a null modem adapter as a last resort but then I didn't see
any light activity either. I have tried this with three different
pieces of hardware so far: a DECserver 700 teminal server with full
modem control capability; a serial port on an AlphaStation and a DEC
terminal. The result is the same with all.

I have not been able to do a software test with a PC yet because all the
IRQ's are in use. However, I did connect the mini tester to a serial
port and I noticed one difference: the TD, RTS and DTR LED's glow, but
red this time, which I believe means negative with respect to ground.

Does anyone have any experience with this or any ideas or suggestions?

Thanks in advance.
Bill McLaughlin

[Bill McLaughlin]

Sorry I didn't check this before...

I am still having the same problem but I do have a little more info. After
I posted the original note, I disconnected from the internet, which freed up
my modem port. I then connected the meter to that port and ran a terminal
emulator program (Reflection 4). After setting the port to 7 bit, 600 baud,
etc. I got the same result as with the other hardware. Then it occurred to
me to try the software that came with the meter. I installed it and
reconnected the meter. At first the LED's were the same: Red on all. But
when I hit the "on" button on the software, the DTR went from red to green.
At this point, the software was able to talk to the meter. So, apparently
the meter either needs to see a negative voltage on RTS or it needs to see a
wider difference (+12 to -12 .vs. +12 to 0) overall.

I don't believe there is any way to get the non-PC hardware to supply the
negative voltages. Does anyone have any suggestions for a way to interface
this? I would prefer an off-the-shelf product but I can build something if
necessary.

Using a PC for this application is not an option.

Thanks again.
Bill McLaughlin

[Thomas Lutz]

If your communications parameters do not match exactly then you will
never get things to work.
If the device uses 7 data bits then it must also use some form of
Parity, Make sure that you have the Parity option set correctly.
One thing that you may need to do is to append a carriage return after
the D command to the meter. Some devices do not respond to commands
until a carriage return is received.

My web site has some good diagnostic software that you can download
for free. Visit http://www.taltech.com and look in the Free Software
section.

[Bill McLaughlin]

Thanks for the info. At this point, I don't believe it is a parity, etc.
problem but rather a signal problem. This is because I don't get any
parity errors or strange characters from the device-- nothing shows at
all. Normally when it is a parity or word size or baud rate issue, I see
garbage. I just got a DT-5 breakout box with *all* the status LED's and
an "Auto-Fox" rs-232 analyzer from Datatran. Hopefully this will solve
the problem. BTW, I had never heard of Datatran before now. A friend
suggested I call them and their service is awesome. I'll reply here when
the problem is solved (or if).

Unfortunately, I am beginning to suspect that the engineers that designed
this meter never really cared about the RS-232 standard but rather just
said "Well, it works with the PC serial port...". The only reason I say
this is that I am stumped so far and I have successfully interfaced a huge
variety of RS-232 devices to this same equipment over the years.

Bill McLaughlin

[Bill McLaughlin]

Well, the problem is solved. Thanks to the DT-5 breakout box, I found the
correct configuration. The Metex meter needed to have a low supply as well as
a high. The low needs to be supplied to pin 4 (RTS). Luckily, the terminal
server I am connecting it to happens to supply a low on pin 23 (DSRS).

I don't know if anyone else here would be running into the same problem
(hooking the meter to a non-PC serial port) but here's all you need:

A straight through cable with lines 2(TD), 3(RD), 7(SG), 20(DTR). Then connect
pin 4(RTS) on the meter to a low supply on the other end: in this case
23(DSRS).

The software configuration was the same as the manual said: 600 baud, 7 data,
2 stop bits, no parity, no handshaking.

Now that this works, it's really pretty cool. For only $39.95 I have a device
that can monitor temperature, voltage, etc. for logging or automatic shutdown
purposes.

Bill McLaughlin