PLC 5/20 - 1785-ENET Rev C, Digi One IAP, MODBUS/RTU

Digi One IA/IAP
1

I have a PLC 5/20 - Series C, Rev N.1 with a 1785-ENET Rev C “piggy back” ethernet card. I’m trying to poll a Modbus/RTU for certain registers (41001-41006). Currently I have my ModSim32 acting as my Modbus Slave for Testing purposes.

Digi Config:
I have my Master set up as Ethernet/IP and Allen-Bradley ethernet, per advice from tech support. I have my Modbus/TCP wired into Port 2 RS232. I have adjusted the jumpers accordingly. The other end of the cable is plugged into my COM port and is successfully connected.

PLC 5 Message Block:
SLC Typed Logical Read
N258:6
Size: 2 (I have 2 additional message blocks each of size 2 to capture all data I need to read)
Port Number: 3A

Data Table Address: N20:0
Multihop: Yes

Multihop Setup (I’ve tried without Multihop and had no success, Rockwell told me that using Multihop is the equivalent of utilizing CIP):
Ethernet Dev.: IP Address of Digi (10.4.25.10)
Backplane Slot: 10 (Modbus ID of virtual slave on ModSim32)

I keep receiving the error:
3000 Remote missing, disconnected, or shut down.

On ModSim32, I’m seeing data coming in constantly when I “show traffic;” however, I see 0 Valid Messages and a ton of Invalid CRCs.

Does anyone know what I’m doing wrong? I’ve been beating my head against the wall for two days straight now. HELP!

2

Sorry I missed this post - bad CRC sounds like serial setting problems. Make sure the baud rate, parity, and no-flow-control is correct. Is you cable just a common null-modem cable? The IAP normally ships with one.

Take a look at this page, which explains how to see an ‘event trace’ of what the DOIAP is doign internally. If ModSim is seeing CRC errors, then it never answers and the PLC5 will always see no response.

Also make sure the ladder-logic driving the MSG block is either manual trigger only, so some slow timer trigger (once per 15 seconds). Tying the MSG enable to the rail can cause concestion and timing problems.
http://digitips.wikispaces.com/Digi_IA_Event_Trace