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// Function declaration for dehex()
byte dehex(char c); // Get nibble value 0...15 from character c
void SendCTRL (String input) {
 const char *hin = input.c_str(); // Get character array
 int clen = input.length/2;
 // Next line invalid in C++, ok in C99. Probably need to
 // instead declare a fixed-length array, cmd[MAXCMDLEN], etc
 char cmd[clen+1]; // Leave a byte for null terminator
 for (int i=0; i < 2*clen; i+=2) {
 cmd[i/2] = dehex(hin[i])<<4 + dehex(hin[i+1]);
 }
 cmd[clen] = 0; // Null-byte terminator
 digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit 
 RS485Serial.write(cmd); // Send string someplace
 delay(clen/4); // Wait for write to complete? What rate?
 digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
}
byte dehex(char c) { // Get nibble value 0...15 from character c
 // Treat digit if c<'A', else letter
 return c<'A'? c & 0xF : 9 + (c & 0xF);
 // Above assumes that c is a 'hex digit' in 0...9, A or a ... F or f.
 // It would make more sense to just use 16 consecutive characters,
 // like eg 0123456789:;<=>? or @ABCDEFGHIJKLMNO so the above
 // could just say `return c & 0xF;`
}

1. The first statement, byte dehex(char c); is a function declaration for the dehex() function. It tells the C/C++ compiler the datatypes that the function accepts and returns. Note, instead of a function declaration at that point, one could instead put the function definition itself (which in the above follows SendCTRL(), a dehex() caller that needs to know dehex()'s datatypes).

2. In this edit I added const before char *hin = input.c_str(); because ATE-ENGE reports "char *hin =input.c_str() is giving me an invalid conversion error from const char* to char*". That error indicates the left-hand-side variable has a datatype different from that of the right-hand-side expression and an automatic conversion isn't available. Note, unfortunately the arduino.cc c_str() webpage doesn't specify the return type of c_str().

Edit 3: Changed for (int i=0; i < clen; i+=2) to for (int i=0; i < 2*clen; i+=2) per notes in another question's answer.

// Function declaration for dehex()
byte dehex(char c); // Get nibble value 0...15 from character c
void SendCTRL (String input) {
 const char *hin = input.c_str(); // Get character array
 int clen = input.length/2;
 // Next line invalid in C++, ok in C99. Probably need to
 // instead declare a fixed-length array, cmd[MAXCMDLEN], etc
 char cmd[clen+1]; // Leave a byte for null terminator
 for (int i=0; i < 2*clen; i+=2) {
 cmd[i/2] = dehex(hin[i])<<4 | dehex(hin[i+1]);
 }
 cmd[clen] = 0; // Null-byte terminator
 digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit 
 RS485Serial.write(cmd); // Send string someplace
 delay(clen/4); // Wait for write to complete? What rate?
 digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
}
byte dehex(char c) { // Get nibble value 0...15 from character c
 // Treat digit if c<'A', else letter
 return c<'A'? c & 0xF : 9 + (c & 0xF);
 // Above assumes that c is a 'hex digit' in 0...9, A or a ... F or f.
 // It would make more sense to just use 16 consecutive characters,
 // like eg 0123456789:;<=>? or @ABCDEFGHIJKLMNO so the above
 // could just say `return c & 0xF;`
}

1. The first statement, byte dehex(char c); is a function declaration for the dehex() function. It tells the C/C++ compiler the datatypes that the function accepts and returns. Note, instead of a function declaration at that point, one could instead put the function definition itself (which in the above follows SendCTRL(), a dehex() caller that needs to know dehex()'s datatypes). Some versions of the Arduino IDE appear to work ok without function declarations ahead of use, but almost all other C/C++ environments will not.

2. In this edit I added const before char *hin = input.c_str(); because ATE-ENGE reports "char *hin =input.c_str() is giving me an invalid conversion error from const char* to char*". That error indicates the left-hand-side variable has a datatype different from that of the right-hand-side expression and an automatic conversion isn't available. Note, unfortunately the arduino.cc c_str() webpage doesn't specify the return type of c_str().

Edit 3: Changed for (int i=0; i < clen; i+=2) to for (int i=0; i < 2*clen; i+=2) per notes in another question's answer.

Edit 4: Changed cmd[i/2] = dehex(hin[i])<<4 + dehex(hin[i+1]) to cmd[i/2] = dehex(hin[i])<<4 | dehex(hin[i+1]) to avoid an operator precedence problem. The precedence of +, addition, is higher than that of <<, bitwise shift, which in turn is higher than that of |, bitwise OR. The unparenthesized expression with + adds 4 to dehex(hin[i+1]) and uses the total as a shift count; not at all as desired. Note, one can of course use parentheses to control the problem, as in the expression (dehex(hin[i])<<4) + dehex(hin[i+1])

// Function declaration for dehex()
byte dehex(char c); // Get nibble value 0...15 from character c
void SendCTRL (String input) {
 const char *hin = input.c_str(); // Get character array
 int clen = input.length/2;
 // Next line invalid in C++, ok in C99. Probably need to
 // instead declare a fixed-length array, cmd[MAXCMDLEN], etc
 char cmd[clen+1]; // Leave a byte for null terminator
 for (int i=0; i < clen; i+=2) {
 cmd[i/2] = dehex(hin[i])<<4 + dehex(hin[i+1]);
 }
 cmd[clen] = 0; // Null-byte terminator
 digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit 
 RS485Serial.write(cmd); // Send string someplace
 delay(clen/4); // Wait for write to complete? What rate?
 digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
}
byte dehex(char c) { // Get nibble value 0...15 from character c
 // Treat digit if c<'A', else letter
 return c<'A'? c & 0xF : 9 + (c & 0xF);
 // Above assumes that c is a 'hex digit' in 0...9, A or a ... F or f.
 // It would make more sense to just use 16 consecutive characters,
 // like eg 0123456789:;<=>? or @ABCDEFGHIJKLMNO so the above
 // could just say `return c & 0xF;`
}

// Function declaration for dehex()
byte dehex(char c); // Get nibble value 0...15 from character c
void SendCTRL (String input) {
 const char *hin = input.c_str(); // Get character array
 int clen = input.length/2;
 // Next line invalid in C++, ok in C99. Probably need to
 // instead declare a fixed-length array, cmd[MAXCMDLEN], etc
 char cmd[clen+1]; // Leave a byte for null terminator
 for (int i=0; i < 2*clen; i+=2) {
 cmd[i/2] = dehex(hin[i])<<4 + dehex(hin[i+1]);
 }
 cmd[clen] = 0; // Null-byte terminator
 digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit 
 RS485Serial.write(cmd); // Send string someplace
 delay(clen/4); // Wait for write to complete? What rate?
 digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
}
byte dehex(char c) { // Get nibble value 0...15 from character c
 // Treat digit if c<'A', else letter
 return c<'A'? c & 0xF : 9 + (c & 0xF);
 // Above assumes that c is a 'hex digit' in 0...9, A or a ... F or f.
 // It would make more sense to just use 16 consecutive characters,
 // like eg 0123456789:;<=>? or @ABCDEFGHIJKLMNO so the above
 // could just say `return c & 0xF;`
}

Edit 3: Changed for (int i=0; i < clen; i+=2) to for (int i=0; i < 2*clen; i+=2) per notes in another question's answer .

byte dehex(char c); // Get nibble value 0...15 from character c
void SendCTRL (String input) {
 char *hin = input.c_str(); // Get character array
 int clen = input.length/2;
 // Next line invalid in C++, ok in C99. Probably need to
 // instead declare a fixed-length array, cmd[MAXCMDLEN], etc
 char cmd[clen+1]; // Leave a byte for null terminator
 for (int i=0; i < clen; i+=2) {
 cmd[i/2] = dehex(hin[i])<<4 + dehex(hin[i+1]);
 }
 cmd[clen] = 0; // Null-byte terminator
 digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit 
 RS485Serial.write(cmd); // Send string someplace
 delay(clen/4); // Wait for write to complete? What rate?
 digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
}
byte dehex(char c) { // Get nibble value 0...15 from character c
 // Treat digit if c<'A', else letter
 return c<'A'? c & 0xF : 9 + (c & 0xF);
 // Above assumes that c is a 'hex digit' in 0...9, A or a ... F or f.
 // It would make more sense to just use 16 consecutive characters,
 // like eg 0123456789:;<=>? or @ABCDEFGHIJKLMNO so the above
 // could just say `return c & 0xF;`
}

// Function declaration for dehex()
byte dehex(char c); // Get nibble value 0...15 from character c
void SendCTRL (String input) {
 const char *hin = input.c_str(); // Get character array
 int clen = input.length/2;
 // Next line invalid in C++, ok in C99. Probably need to
 // instead declare a fixed-length array, cmd[MAXCMDLEN], etc
 char cmd[clen+1]; // Leave a byte for null terminator
 for (int i=0; i < clen; i+=2) {
 cmd[i/2] = dehex(hin[i])<<4 + dehex(hin[i+1]);
 }
 cmd[clen] = 0; // Null-byte terminator
 digitalWrite(SSerialTxControl, RS485Transmit); // Enable RS485 Transmit 
 RS485Serial.write(cmd); // Send string someplace
 delay(clen/4); // Wait for write to complete? What rate?
 digitalWrite(SSerialTxControl, RS485Receive); // Disable RS485 Transmit
}
byte dehex(char c) { // Get nibble value 0...15 from character c
 // Treat digit if c<'A', else letter
 return c<'A'? c & 0xF : 9 + (c & 0xF);
 // Above assumes that c is a 'hex digit' in 0...9, A or a ... F or f.
 // It would make more sense to just use 16 consecutive characters,
 // like eg 0123456789:;<=>? or @ABCDEFGHIJKLMNO so the above
 // could just say `return c & 0xF;`
}

Edit 2:

1. The first statement, byte dehex(char c); is a function declaration for the dehex() function. It tells the C/C++ compiler the datatypes that the function accepts and returns. Note, instead of a function declaration at that point, one could instead put the function definition itself (which in the above follows SendCTRL(), a dehex() caller that needs to know dehex()'s datatypes).

2. In this edit I added const before char *hin = input.c_str(); because ATE-ENGE reports "char *hin =input.c_str() is giving me an invalid conversion error from const char* to char*". That error indicates the left-hand-side variable has a datatype different from that of the right-hand-side expression and an automatic conversion isn't available. Note, unfortunately the arduino.cc c_str() webpage doesn't specify the return type of c_str().