22 FP-550F
B) Non-wrapped messages - TIME-OUT
When the transmitting of messages from the Host is normal, Slave answers not later
than 60 ms either with a wrapped message or with a 1 byte code. Host must have 500
ms of time-out for receiving a message from Slave. If there is no message during this
period of time the Host will transmit the message again with the same sequence num-
ber and the same command. After several unsuccessful attempts Host must indicate
that there is either no connection to the fiscal printer or there is a hardware fault.
Non-wrapped messages consist of one byte and they are:
A) NAK 15H
This code is sent by Slave when an error in the control sum or the form of the
received message is found. when Host receives a NAK it must again send a message
with the same sequence number.
B) SYN 16H
This code is sent by Slave upon receiving a command which needs longer pro-
cessing time. SYN is sent every 60 ms until the wrapped message is not ready for
transmitting.
C) Wrapped messages
a) Host to printer (Send)
<01><LEN><SEQ><CMD><DATA><05><BCC><03>
b) Printer to Host. (Receive)
<01><LEN><SEQ><CMD><DATA><04><STATUS><05><BCC><03>
Where:
<01> Preamble
1 byte long
value: 01H
<LEN> number of bytes from <01> preamble (excluded) to <05> (included) plus the
fixed offeset of 20 H.
length: 1 byte
value: 20H - 7 FH
<SEQ> Sequence number of the frame
length : 1 byte
value: 20H - F FH
The fiscal printer saves the same <SEQ> in the return message. If the FP gets a mes-
sage with the same <SEQ> as the last message received it will not perform any opera-
tion, but will repeat the last sent message.
<CMD> The code of the command
length: 1 byte
value: 20H - 7FH
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