You must keep track of the number of received passings to calculate the correct [StartId] for the next PASSINGGET call.

Example 1 - Empty passing buffer at start

← PASSINGGET;00000000\n       <1>
→ PASSINGGET;00\n             <2>
→ 00000000;00\n               <3>
→ \n[...some time later, after creating passings...]
← PASSINGGET;00000000\n       <4>
→ PASSINGGET;00\n             <5>
→ 00000000;03\n               <6>
→ GLBAS60;0718;01521527;0c;08;9f;1a;0;1;2;00;0\n
→ GLBAS70;04c1;01521536;14;09;9f;1a;0;1;2;00;0\n
→ EMPAL70;047c;0152153b;0e;08;9f;1a;0;1;2;00;0\n
→ \n
[... small wait between requests ...]
← PASSINGGET;00000003\n       <7>

1. Try to recieve passings starting at 0x00000000,…
2. …no error code
3. …no passings available ? [StartID] still 0x00000000
4. Try to receive passings again
5. …still no error
6. …we have 0x03 passings, starting with ID 0x00, following in the next lines.
7. Next try, receive passings from [StartID]==0x00000003 onwards

Example 2 - More than 64 passings

← PASSINGGET;00000000\n
→ PASSINGGET;00\n
→ 00000000;40\n               <1>
→ GLBAS60;0718;01521527;0c;08;9f;1a;0;1;2;00;0\n
→ GLBAS70;04c1;01521536;14;09;9f;1a;0;1;2;00;0\n
[...]
→ SNRMT06;1260;015215d8;0d;0c;9f;15;0;1;2;01;0\n
→ SNRMT07;2910;015215c9;30;0f;9f;15;0;1;2;01;0\n
→ \n

1. Number of passings sent in this response is 0x40 == 64. We should call the command again as there may be more passings which have not been sent.
   

   ← PASSINGGET;00000040\n       <2>
   → PASSINGGET;00\n
   → 00000040;05\n               <3>
   → GLBAS98;04d3;015215c9;11;08;9f;1a;0;1;2;01;0\n
   → GLBAS58;074b;0152154d;11;0a;9f;1a;0;1;2;03;0\n
   → SNRMT29;2ad4;01521590;47;0f;9f;15;0;1;2;02;0\n
   → GLBAS78;04e0;01521594;0f;09;9f;1a;0;1;2;02;0\n
   → SNRMT39;111e;01521594;2e;0e;9f;15;0;1;2;04;0\n
   → \n
   

2. Read further passings, starting with 0x40

3. This transmission only contains 0x05 passings => buffer fully read

Example 3 - Buffer overflow or StartID too high

← PASSINGGET;00000000\n
→ PASSINGGET;10\n             <1>
→ 00000000;0000021d\n         <2>
→ \n

1. Error 10 StartID out of range. Passing 0x00 (original [StartID] in query) has been deleted as the buffer has overflowed.
2. The first available passing has the ID 0x21d = 541, so there are passings 0x21d = 541 to 0x21d + 0d1000 = 0x604 = 1540 available.