modicon stumped newbie question


Thread Starter


Hello. I bought a MODICON 612 at a garage sale for learning purposes as I cannot go to school and am not getting very far. I have searched this subject and have had about as much success. I have the software set up on a comp with windows 98 and have the unit powered up but when I try to send a program I get "modbus checksum error 20". I used to get error 199 but fixed that but cannot get around this one no matter what I do. I have it configured right and the com1 light just flickers before returning this error. I tried chaning the serial cord to no avail. I realize this thing is as old as the hills and this is a stupid beginner question but I cannot figure it out. Maybe this thing is fried to begin with? I have no problem programing thru the lpt but this is different.

Any help appreciated.


James Fountas

Hi Scott,

What software are you using? A typical glitch I have run across is newer (and not so old) PCs can have issues talking Modbus RTU through a comm port. I have switched the PLCs from Modbus RTU to Modbus ascii and newer PCs ususally start talk. The problem is you have to be able to talk to the PLC before you can switch it to Modbus ascii. This may or may not be your problem. Feel free to e-mail me.


James Fountas
[email protected]

Bradley Hite

<p>Hello Scott,

<p>Unless you have the correct cable pinouts nothing is going to work.

<p>Here is the cable pinout that I use:

<p>PC End for Micro and Momentum controllers
RJ45 9 Pin
Pin Color Female End
2 ---- Orange------- 4--|
3 -----Black-------- 2
4 -----Red---------- 3
5 -----Green-------- 5
6 -----Yellow--|
7 -----Brown---| 7--|

<p>RJ45 Cable is a straight through cable.
| |
| |
| |
-- --
pin | | pin
1 - - 8
| |
<p>The software communication settings for a Modbus port set to default
settings are 9600,8,E,1.

<p>To return the processor to default settings disconnect the battery and
remove the processor from power.

<p>With the correct cable and the above comm settings you should be able to
communicate with the processor.

<p>Feel free to contact me directly if you want.

<p>Bradley G. Hite<br>
Intertech Inc<br>
mailto:[email protected]<br>
Teaching Practical Skills for a Technological World