| The Status byte contains basic information about the state of the module, including whether or not the previous command had a checksum error. The specific bit definitions for the Status byte are in Section 5.3 below. The number Additional Status Data bytes is programmable, and may contain information such as motor position, A/D values, or the module type and version numbers. Exactly which data is included in these Additional Status Bytes can be programmed using the Define Status or Read Status commands. On power-up or reset, each NMC module defaults to sending only the Status byte and Checksum byte, with no additional status data. A command sent to a PIC-SERVO controller is stored in an internal buffer until the end of the current servo cycle (0.51 millisec. max.), when it is then executed and a status packet is returned. Therefore there will be a maximum delay of 0.51 millisec. (0.25 millisec. avg.) between when the last byte of the command is received and when the first byte of the status packet is sent. No new command should be sent until the status packet is returned to prevent overwriting the command data buffer and to prevent collisions on the status line. If, however, the host does send any data before a status packet is received, all slaves on the network will disable any status data transmission in progress and listen to the new command from the host. This insures that the host can always command the attention of all slaves on the network. The Command Reference Section 5.1 below describes the data contained in the command packets and status packets. Addressing When multiple modules are connected to the same NMC network, they must be assigned unique addresses. This is done through the use of the ADDRJN and ADDROUT signals on each NMC compatible controller. The ADDROUT signal from one controller is daisy-chained to the ADDRJN signal of the adjacent controller on the network. Customarily, the ADDRJN pin of the controller furthest from the host is tied to GND, and the ADDROUT signal of the controller closest to the host is left open. (See the Figure 2 above). Unique addresses are assigned using the following procedure: 1. On power-up, all modules assume a default address of 0x00, and each will set its ADDROUT signal HIGH. Furthermore, a module's communications will be disabled completely until its ADDRJN signal goes LOW. If the ADDROUT and ADDRJN signals are daisy-chained as described above, all modules will be disabled except for the module furthest from the host. 2. The host starts by sending a Set Address command to module 0, changing its address to a value of 1. A side affect of the Set Address command is that the module will lower the its ADDROUT signal. 3. At this point, the next module in line is enabled with an address of 0. The host then sends a command to module 0 to change its address to a value of 2. 4. This process is continued until all modules have been assigned unique addresses. Initialization of the addresses is performed by the host each time the NMC network is powered up or reset. The host can also use this mechanism to verify that the proper number of modules are present, and that their types match those expected for a particular application. Once addresses are set, all other operations can be executed. JEFFREY KERR, LLC • www.jrkerr.com 6 |