刚才又试了一下，第二个和第三个问题已经解决
安装后找不到帮助文件是因为安装的路径有问题，没有能够安装到到STEP7安装路径下的BIN文件夹中，重新选择路径后可以正常调出帮助文件
打开帮助文件后，帮助文档如下：
Bytes 1208 to 1223:
Failed (inaccessible) slaves as bit register



Bytes 1224 to 1239:
Faulty slaves as bit register



Bytes 1240 to 1255:
Affected slaves (failed or faulty) as bit register



Bytes 1256 to 1271:
Affected slaves stored as bit register, i.e. incoming slave diagnoses (faulty or failed) are stored in this bit register until the bit register is deleted again via CPU complete restart or block RESET.



Bytes 1272 to 1397:
Number of diagnostics per slave (Slave1: Byte 1272 ... Slave126: Byte 1397)

发现可以访问上述DB块中数据进而知道所有出故障的节点地址

现在还剩下一个问题，FB125 是否支持对CP342-5的诊断？