Yaskawa MP920 Communications Module - Manual (Page 76)

5.4 Cables
5-15
5
5.4.4 Tokens
„ Overview of the Communications
The time from when a token is received until the next time the token is received is called the
token cycle time. The time during which the right to send is obtained, that is, the time from
when the token is received until the token is transmitted, is called the token holding time.
215IF communications operate with the upper limit of the token holding time being the dif-
ference in time between the token cycle time (set value) set in the transmission parameter
and the token cycle time (current value) measured at each receipt of the token. Link trans-
mission data is transmitted even when this upper limit is exceeded, whereas message trans-
mission data is not transmitted but is held if there is any possibility that the upper limit may
be exceeded.
„ Estimation of the Token Cycle Time
The token cycle time at each station can be estimated using the equations given below.
In each equation, the message transmission margin refers to the time margin for message
transmission from a station included in the token cycle time. A minimum message transmis-
sion of 1.2 ms for 4 Mbps, 2.4 ms for 2 Mbps, or 4.8 ms for 1 Mbps should be guaranteed.
If this value is increased, the message transmission efficiency will increase, but link trans-
mission data exchanges will become slower.
• Transmission speed: 4 Mbps
Transmission cycle (ms)
= 0.16
× station No. + 0.004 × total number of link transmission words
+ message transmission margin ( ≥ 1.2 ms)
• Transmission speed: 2 Mbps
Transmission cycle (ms)
= 0.23
× station No. + 0.008 × total number of link transmission words
+ message transmission margin ( ≥ 2.4 ms)
ST#64
Token
addressed
to ST#1
Token
addressed
to next ST#
ST#1
Preamble
ST#2 ST#3 ST#64 ST#1
Token cycle time of ST#1 (current value)
(From receipt of a token until receipt of the next token)
Token holding time of ST#1
(From receipt of token until transmission of token)
Upper limit of
token holding time
Token cycle time of ST#1 (set value)
Link transmission data
(Broadcast to all ST#) Message
transmission data