Performance Motion Devices PR8358120, User Manual

Page: 66 / 103

Accessing Card Resources
66
Prodigy/CME PC/104 User’s Guide
3
3.5 PRP Communication Formats
The following sections discuss how PRP messages should be formatted on the Serial, CANbus, and Ethernet
networks so that they can be properly interpreted by the Prodigy/CME PC/104 card. Note that PRP messages from
a host are not sent over the PC/104 bus, because the Prodigy/CME PC/104 card acts as the bus master.
Communication via PRP from a host to the Prodigy/CME PC/104 card occurs via Serial, CANbus, or Ethernet.
3.5.1 PRP Messages over Serial
The Prodigy/CME PC/104 card can receive PRP command messages from a host controller on Serial port 1 or Serial
port 2. This section describes the format of these packets, which transfer PRP messages over a serial protocol.
Figure 3-6 shows the Serial packet protocol that must be used for all PRP messages. This serial header is 32 bits, and
is broken into three fields as follows:
Address : the first byte is an address field, used only with RS485 communications. For RS232 communications, this
byte is not included in the packet.
Checksum : The second byte is a simple 8-bit sum checksum of all bytes in the packet payload (excluding the serial
header, i.e., the address, checksum, and length fields). From Figure 3-6 this means only the PRP header and message
body contribute to this checksum value
Length : The third byte is an 8 bit length field. The length indicates the number of bytes in the packet payload. The
packet payload is defined as everything in the serial packet after the length field. That is, the 2 byte PRP header plus
however many bytes are contained in the PRP message body. For example if the PRP message body is 6 bytes in size,
the value filled into the length field should be 8 (2 bytes for PRP 6 bytes for the PRP message body).
Figure 3-6:
PRP Message
over Serial
Format
Return PRP messages should be formatted in the same way.
3.5.1.1 Serial packet processing
An error-free Serial/PRP communication sequence from the host controller to the Prodigy/CME PC/104 card
consists of a full outgoing packet transmission with the correct checksum and specified number of bytes received by
the Prodigy/CME PC/104 card, and a full packet response with correct checksum and length received at the host
controller. The return message must be received within a fixed amount of time determined by the host controller.
Correctly setting this ‘timeout window’ may depend on factors such as baud rate, but 100 mSec is a typical safe value.
If the host controller receives a response packet with an incorrect checksum, or does not receive the complete
response (communications timeout), then the original message should be resent.
If the Prodigy/CME PC/104 card receives a packet with an incorrect checksum, then a packet with an error code
indicating this is returned to the host controller.
If the Prodigy/CME PC/104 card does not receive the specified number of bytes within 100 mSec (the Prodigy/CME
PC/104 timeout value) of beginning of packet reception, the incoming message is ignored, and no message is sent to
the host controller.
Address Checksum Length
1 byte 1 byte 1 byte
PRP Message
(RS485 only)