10 TROUBLESHOOTING
10.1.4 Reading the data communication status (Transmission sequence status)
This section explains how to read the current status of the data communication using the MC protocol stored in the buffer memory.
When GX
When a transmission problem occurs, read operation is executed to check the status of the data communication using the MC protocol.
(1) Transmission sequence status storage area (address: 255H and 265H)
The status of data communication using the MC protocol is stored as a numerical value in buffer memory addresses 255H and 265H.
| b15 | to | b0 | |
Buffer memory address 255H |
| 0 to 9 |
| (Information on the CH1 side) |
265H | 0 to 9 |
| (Information on the CH2 side) | |
The following shows the correspondence between the numerical values in the transmission sequence status storage area and the data communication status.
Request from an external device
E
External N
device Q
Q series C24
Station number HL
Message wait
ACK
or
NAK
Request
to CPU
Response |
| |
from | "6" "1"..."6" is repeated afterward. | |
CPU | ||
|
Stored | 0 | 1 | 3 | 4 | 5 | 6 |
value |
|
| 2 | Values 7 to 9 are stored when "mode switching" or "initialization | ||
of transmission sequence" is performed.
REMARK
When the target interface is not set to use the MC protocol, "0" is stored in the transmission sequence status storage area.
(2)Example of a program that reads from transmission sequence status storage area
The following shows an example of a program that reads from the transmission sequence status storage area.
(input/output signals X/Y00 to X/Y1F of the Q series C24) 
Example of FROM instruction
Read command
b15to
D00
D14
Reads the status of data communication on the CH1 side from address 255H.
Reads the status of data communication on the CH2 side from address 265H.
b0
The CH1 side is waiting to receive a command message, and is using non procedure or bidirectional protocol.
CH2 is accessing the PLC CPU upon reception of a command message.
10 - 10 | 10 - 10 |