EM1500 User’s Manual
97
The “Rx idle time” field specifies how long the serial port receiver must be idle for in order to declare end-
of-packet. The “Tx idle time” is the minimum amount of time that the serial port transmitter must be idle
between packets. Both of these values would normally be the same; however, they can be set indepen-
dently to support asymmetric timeouts.
Note that the Rx idle time value is rounded up to the nearest multiple of 1/32768 seconds (i.e., about 30
µs). This is because the incoming data is time stamped with this clock resolution.
The Tx idle time is rounded up to a multiple of 1/2048 seconds (i.e., 488 µs) since that is the resolution of
the clock which determines when to transmit the next character (after the transmitter becomes idle).
When transmitting a packet, the EM1500 will always transmit every character back-to-back, i.e., with no
inter-character gap, provided that the packet size is less than or equal to 1020 bytes. When receiving, inter-
character gaps are allowed provided that they are shorter than the specified Rx idle timer.
B.2.1.2 String Packetization
There is no length limitation on any packet when using string packetization. This is from the point of view
of the serial device and is unrelated to the length limitation imposed on a TCP packet.
Packets are terminated when a particular character sequence is received. The string to match may be of any
length between 1 and 8 characters, however it will typically be 1 or 2 characters. Most commonly a car-
riage return and/or line feed character is used to signal the end of a packet.
When data is being received from the serial port, the EM1500 is looking for a sequence of characters that
exactly matches the terminator string. When the string is seen, one of two things happens depending on
which version of string packetization is selected.
1. If “String - send” is selected, then the terminating string is sent over the network, followed by the EOP
indicator.
2. If “String - cut” is selected, then the EOP is sent but without the terminating string itself.
In the other direction, when an EOP indicator is received from the network, the EM1500 takes the follow-
ing action when transmitting the packet out the serial port:
1. If “String - send”' is selected, then it basically does nothing (since the terminating string of the packet
has already been received from the network).
2. If “String - cut” is selected, then the EM1500 inserts the terminating string and sends it out the serial
port. This is necessary because the network peer cut out the terminating string before sending the EOP.
Summary of Contents for EM1500
Page 14: ...10 www rabbit com Introduction...
Page 22: ...18 www rabbit com Getting Started...
Page 76: ...72 www rabbit com EM1500 Configuration...
Page 90: ...86 www rabbit com EM1500 Specifications...
Page 104: ...100 www rabbit com Serial and TCP Protocols...
Page 118: ...114 www rabbit com EM1500 FAQ...