Envoy ACP/XF Application Programming Manual

Normal Response Mode (NRM) Protocols

EnvoyACP/XF Application Programming Manual–132179

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SDLC and ADCCP-NRM Error Handling

EnvoyACP/XF returns two general types of errors: line errors and link errors.

The application program must ensure the end-to-end transfer of application-generated,

unnumbered frames by issuing a READ call (or using WRITEREAD) to receive the

response, then check the validity of the response.

For example, when using WRITEREAD (or a WRITE followed by a READ) to send the

SNRM, SNRME, SIM, or DISC command, the application must examine the contents of

the READ buffer to be sure that EnvoyACP/XF received a UA response. The

error-recovery method used with CONTROL 13, 14, and 18 auto-polling does not apply

if you are sending the SNRM, SNRME, SIM, or DISC commands by using individual

WRITE or WRITEREAD calls.

Line Actions by Procedure Call

EnvoyACP/XF performs the actions described below in response to user-application

calls to Guardian file-system procedures.

NRM READ Procedure

EnvoyACP/XF accumulates all incoming frames in a dedicated input buffer. If any

received frames are present already in this buffer, each READ request obtains the oldest

frame and passes the pertinent data and control information back to the application

program.

Line

errors

These are errors such as garbled transmission.

EnvoyACP/XF retransmits the I-frame up to the retry threshold before

informing the application. If a WRITE operation is retried the configured

number of times without receiving the proper response, EnvoyACP/XF

removes the WRITE request and all subsequent pending WRITE requests

to the same station from the queue and completes them with a CCL and

file-system error 162.

Link

errors

These are communication errors such as the receipt of an invalid

command or response from the remote station.

EnvoyACP/XF informs the application process immediately when there is

a link error. Higher-level recovery is the responsibility of the application

process. The primary-station application process can re-establish the link

(send the SNRM) by using the CONTROL or WRITE procedures. The

secondary-station application can accept commands by using READ

requests, and generate direct responses by setting the MCW on WRITE

requests.

Note. Error recovery for unnumbered frames does not guarantee that the remote station will

receive the frame.