Rane ECM 64 - Manual (Page 25)

Manual-25
Example Packet Expansion Code for RW 232 Messages
// 09-10-96 - Devin Cook (Derived from RW232.CPP code)
// This code only deals with the Body of an RW 232 message (Command/Data)
// The steps needed to fully communicate with an RW 232 device are as follows:
// 1. Send the Address: [FB xx FB xx]
// 2. Get the returned Device Type and Device ID flags
// 3. Send the FB expanded Body
// 4. Get and check the returned ComStat byte
// Take a simple command and expand it into a full packet.
// Input:
// Buff - BYTE array with the unexpanded message and lots of extra room
// MsgLen - Unexpanded message length
// Steps required are:
// 1. Add Packet size. This is simply the Command length + 1 for the checksum
// 2. Duplicate 0xFBs
// 3. Calculate Checksum
// 4. Add Checksum to packet (Check for a 0xFB Checksum!)
// 5. Copy Packet back to the buffer
// 6. Return the new Packet Size
// Note: The buffer must be large enough to accept the expanded data. No checking is done to verify it is, so be careful!
// A packet into this routine consists of the one byte Command and any Data
int CmdToPacket(BYTE Buff[ ], int MsgLen)
{
BYTE L_MSB = ((MsgLen+1) >> 8) & 0xFF; // Grab MSB of Size
BYTE L_LSB = (MsgLen+1) & 0xFF; // Grab LSB of Size
// FBs is the number of 0xFB bytes in the messages
int FBs = 0;
// Don’t forget to check message length for FBs
if (L_MSB == 0xFB)
FBs ++;
if (L_LSB == 0xFB)
FBs ++;
// Calculate Checksum of Message Length bytes along with bytes in the packet
int CheckSum = L_MSB + L_LSB;
for (int x=0;x<L;x++)
{
CheckSum += Buff[x];
if (Buff[x] == 0xFB)
FBs ++;
}
CheckSum = (256-CheckSum) & 0xFF;
// Don’t forget to up the FB count for a FB checksum!
if (CheckSum == 0xFB)
FBs ++;
// New Length is: 2 (For 2 length bytes) + L (Old message Length) + Repeated FBs count + 1 (For CheckSum)
int NewLen = 2 + MsgLen + FBs + 1;
// Create a temporary holding tank for Packet Expansion
BYTE Packet[MAX_CMD_BUF];
BYTE *Ptr = Packet;
// Stick message length in the packet (Watching for FBs of course)
*(Ptr++) = L_MSB ;
if (L_MSB == 0xFB)
*(Ptr++) = 0xFB;
*(Ptr++) = L_LSB ;
if (L_LSB == 0xFB)
*(Ptr++) = 0xFB;
// Expand the original packet into the new buffer
for (x=0;x<L;x++)
{
*(Ptr++) = Buff[x];
if (Buff[x] == 0xFB)
*(Ptr++) = 0xFB;
}
// Add the Checksum byte (or Bytes if Checksum == FB)
*(Ptr++) = CheckSum ;
if (CheckSum == 0xFB)
*(Ptr++) = 0xFB;
// Copy the expanded packet back into the original buffer
memcpy(Buff,Packet,NewLen);
return NewLen;
}