Which are the delays that must be taken into account in connection with serial protocols?

Many serial protocols (e.g. 3964R) work such that the sent data blocks are acknowledged by the respective receiver, such as with the characters ACK or NACK. The time for this acknowledgement is monitored by means of a timeout (ADT = acknowledgement delay time).

To establish this time, the delays on the transmission line as well as in the serial receiver are of interest.

After the data block is sent by the software of the master to the serial driver, the following delays must be added for the dimensioning of ADT for a serial direct connection:

ADT = serial transfer time of data block + serial transfer time of data block + processing time at the slave

Example:

• Length of data block: 100 bytes
• Length of acknowledgment: 1 byte
• Serial data rate: 9600 baud, no parity, 1 start bit, 8 data bits, 1 stop bit = ~ 1 ms / byte
• Processing time at serial terminal device: 2ms
• Minimum ADT: (100 bytes + 1 byte) + 2ms = ~103ms

If a serial master and slave are linked over the network and a Com-Server, the transfer time of typically 4 to 7ms for communication in both directions (option “network delay” = 0) that is required by the Com-Server, must be added to the above value. In addition, delays within the network must also be taken into account. If the following conditions are met, a typical value of approx. 1 to 2 ms can be assumed:

• Network load < 30%
• No ARP address resolution, i.e. existing TCP connection
• no load-disconnecting components (switches, routers, etc.)

Compared with the above described example of a serial direct connection, the combination of network and Com-Server would result in a typical additional delay of approx. 10-18ms:

(2 x 4-7ms for Com-Server) + (2 x 1-2ms for Netzwerk)

Additional information:

• Product Info: Com-Server
• Manual for Com-Server

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