Radio Shack TRS-80 Model II Reference Manual page 93

Library Commands
Required Data Format
The transmitting program must send the data in "Intel
®
Hex Format' , , described
below.
Each byte of data is sent as a pair of hexadecimal ASCII-coded characters:
1)
high nibble (most significant four bits), sent as first byte of pair.
2) low nibble (least significant four bits), sent as second byte of pair.
For example, the value
X'FT
is sent as two bytes, "F" (X'46') followed by' '7"
(X'3T).
Because only":" and ASCII coded hexadecimal numbers are sent, data is always in
the range [X'30', X'3A' ] or [X'41', X'46']. Values outside this range will terminate
reception and produce an error message.
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