User's Manual

PIN Pad 791 Programmer’s Manual (PCI POS-A) UDN PM0103-U Rev. 06
Section 4 – Message format 2015-04-20
Page27
Uniform Industrial Corp. Proprietary and Confidential Total 342 pages
Message frame summary
Data exchanged between PIN Pad and host computer are grouped into “message frames”. Each
message frame has one of the two frame formats listed below:
 <STX>[message ID][data]<ETX>[LRC]
 <SI>[message ID][data]<SO>[LRC]
Each type of message has a unique value in its message ID field. In the following texts, we reference a
message type by its message ID value, e.g. “message 70”.
Message type
Messages exchanged between the PIN Pad and the HOST can be divided into two categories.
One is for administration and maintenance, in general administrative messages have <SI> packet
header and will return message to HOST by the same message ID.
The other is for various transactions, in general transaction messages have <STX> packet header, and
comes in pair. Even number message packets sends command and data to PIN pad, then odd number
message packets returns the result.
Time-out
Whenever the PIN Pad sends a message, a response (<ACK> character for acknowledgement or
<NAK> character if LRC error occurred) from host is expected. If the PIN Pad does not receive a
response within 5 seconds, it will retransmit the last packet. If PIN pad does not receive <ACK> or
<NAK> after two retransmit attempts, it will send <EOT> character and this communication session will
be terminated.
Transmission Error
The PIN Pad expects the host computer to send a NAK when the host decides that an invalid frame is
received. When the PIN Pad receives a NAK, it will retransmit its last message. If the message
retransmitted is invalid again, then a NAK should be sent by host to request for another try. The PIN Pad
will keep on retransmitting until an <ACK> or <EOT> is received.
Packet Error
When PIN pad received a good transmission but invalid packet (wrong message id) it will ignore the
packet. If the packet has acceptable message id but have wrong format. PIN pad will send <EOT> as
error message. When in PIN entry functions it will send more detail error code.