531X170TBSACG1 GE Programmable controller Brand new

Client Channels TCP resource 

management

The OS Network stack hangs on to the TCP resources associated with a 

connection for a period of time after the connection is closed. This time 

period applies to the initiator of the close, which is almost always the client 

side. This time is referred to as the “TCP Linger Period”. Once the TCP 

Linger Period expires (60 seconds in the current OS implementation), the 

TCP resources are released. Application developers using client channels 

need to be aware of this behavior when designing their logic. A finite 

number of TCP resources are allocated to client channels, and if channel 

connections are brought up and down so fast that these resources are 

depleted, then the application may have to wait until a TCP resource frees 

up in order to establish another client channel (a COMMREQ Status of 

0xA890 is returned if no TCP resources are currently available; 

application should wait and try again).

SRTP Client Channels provides features that help the user preserve TCP 

connections. These include a period time where one can establish an 

SRTP Channel and specify the channel to run at a given interval, or run 

as fast a possible. One can also specify a number of iterations, or allow 

the channel to run forever. Additionally, SRTP Channels allows channel 

re-tasking of an active channel to the same remote device, where the 

parameters of an active channel, such as changing the channel command 

type (Read/Write), number of repetitions, time periods, local memory 

address, remote memory address, etc. can be changed. SRTP Channels 

also allows channel re-tasking of an active channel to a different remote 

device (changing the remote device’s IP address, etc.). However, retasking to a different remote device will neither conserve TCP 

connections, nor save on the time it takes to create a channel.

Client Channels and Redundant IP In a Redundancy System, Client Channel COMMREQs can only be 

initiated from the unit that owns the Redundant IP address. Therefore, the 

user application logic should use Bit 6 in the LAN Interface Status bit area, 

“Redundant IP Active” as part of their enabling logic that drives a client 

channel COMMREQ.

An attempt to open 17 or more Modbus 

server connections may appear 

successful

If more than the maximum 16 supported Modbus Server Connections are 

attempted, the TCP connection may succeed, but no data may be 

subsequently transferred.